feat: mic passthrough — client microphone → host virtual PipeWire source

The inverse of the host→client audio path: the client's mic, Opus-encoded, rides a
new 0xCB QUIC datagram to the host, which decodes it into a virtual PipeWire
Audio/Source its apps can record from (voice chat, etc.).

Protocol (punktfunk-core):
- MIC_MAGIC 0xCB + encode/decode_mic_datagram (mirror of the 0xC9 audio datagram).
- NativeClient::send_mic(seq, pts_ns, opus) over a new outbound channel + worker task
  (mirror of send_input); C ABI punktfunk_connection_send_mic for native clients.

Host:
- audio::VirtualMic + PwMicSource: a PipeWire output stream tagged media.class=
  Audio/Source (Direction::Output) — a recordable microphone node, fed decoded PCM.
- MicService: host-lifetime owner of the source + Opus decoder (mirror of
  InjectorService / the audio capturer slot); lazily opened, persists across sessions,
  self-heals. The per-session datagram reader now demuxes 0xCB→mic / 0xC8→input over a
  single read_datagram loop (two loops would race).
- Adaptive jitter buffer in the producer: primes to ~3 consumer quanta before emitting,
  so the 5 ms push / N ms pull clock skew never underruns — without it ~58% of output
  was silence; with it, glitch-free across consumer quanta.

Client: punktfunk-client-rs --mic-test streams a synthetic 440 Hz Opus tone as the mic
uplink (opus dep added) for end-to-end validation without a real microphone.

Validated live on headless KWin: client tone → host source → pw-record shows the
punktfunk-mic Audio/Source node, 440 Hz dominant (Goertzel power 20.7 vs <0.001
elsewhere), RMS 0.179 ≈ the ideal 0.177, 0.3–0.4% silence at both 256 ms and 10 ms
consumer quanta. Tests +1 (mic datagram roundtrip); workspace green, clippy/fmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1953,6 +1953,7 @@ name = "punktfunk-client-rs"
 version = "0.0.1"
 dependencies = [
  "anyhow",
+ "opus",
  "punktfunk-core",
  "quinn",
  "tokio",

crates/punktfunk-client-rs/Cargo.toml

@@ -13,5 +13,6 @@ punktfunk-core = { path = "../punktfunk-core", features = ["quic"] }
 quinn = "0.11"
 tokio = { version = "1", features = ["rt-multi-thread", "net", "time", "macros"] }
 anyhow = "1"
+opus = "0.3"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }

crates/punktfunk-client-rs/src/main.rs

@@ -7,7 +7,9 @@
 //!   stamps each frame with its capture wall clock; same-host runs share that clock).
 //!
 //! `--input-test` exercises the input plane: scripted mouse/keyboard datagrams during the
-//! stream (watch them land in the host session, e.g. xev inside gamescope).
+//! stream (watch them land in the host session, e.g. xev inside gamescope). `--mic-test`
+//! exercises the mic uplink: a synthetic 440 Hz tone streamed as Opus (0xCB) → the host's
+//! virtual microphone source (record it host-side to hear the tone).
 //!
 //! `--pin <64-hex>` pins the host's certificate fingerprint (the host logs it at startup);
 //! without it the client trusts on first use and prints the observed fingerprint to pin.
@@ -37,6 +39,8 @@ struct Args {
     mode: Mode,
     out: Option<String>,
     input_test: bool,
+    /// `--mic-test` — stream a synthetic 440 Hz tone as the mic uplink (proves the mic path).
+    mic_test: bool,
     pin: Option<[u8; 32]>,
     /// `--remode WxHxFPS:SECS` — request this mode SECS seconds into the stream.
     remode: Option<(Mode, u32)>,
@@ -137,6 +141,7 @@ fn parse_args() -> Args {
         mode,
         out: get("--out").map(String::from),
         input_test: argv.iter().any(|a| a == "--input-test"),
+        mic_test: argv.iter().any(|a| a == "--mic-test"),
         pin,
         remode,
         pair: get("--pair").map(String::from),
@@ -348,6 +353,49 @@ async fn session(args: Args) -> Result<()> {
         });
     }
 
+    // Mic plane: stream a synthetic 440 Hz tone as the mic uplink (0xCB), Opus-encoded 5 ms
+    // stereo frames — proves client→host mic passthrough end to end without a real microphone
+    // (the host decodes it into its virtual PipeWire source; record that source to hear the tone).
+    if args.mic_test {
+        let conn2 = conn.clone();
+        tokio::spawn(async move {
+            let mut enc =
+                match opus::Encoder::new(48_000, opus::Channels::Stereo, opus::Application::Voip) {
+                    Ok(e) => e,
+                    Err(e) => {
+                        tracing::error!(error = %e, "mic-test: opus encoder init failed");
+                        return;
+                    }
+                };
+            let _ = enc.set_bitrate(opus::Bitrate::Bits(64_000));
+            tracing::info!("mic-test: streaming a 440 Hz tone as the mic uplink");
+            let mut phase = 0.0f32;
+            let step = 2.0 * std::f32::consts::PI * 440.0 / 48_000.0;
+            let mut pcm = [0f32; 240 * 2]; // 5 ms stereo
+            let mut out = [0u8; 4000];
+            let mut interval = tokio::time::interval(std::time::Duration::from_millis(5));
+            for seq in 0u32.. {
+                interval.tick().await;
+                for f in 0..240 {
+                    let s = (phase.sin()) * 0.25;
+                    phase += step;
+                    if phase > std::f32::consts::PI * 2.0 {
+                        phase -= std::f32::consts::PI * 2.0;
+                    }
+                    pcm[f * 2] = s;
+                    pcm[f * 2 + 1] = s;
+                }
+                if let Ok(n) = enc.encode_float(&pcm, &mut out) {
+                    let d = punktfunk_core::quic::encode_mic_datagram(seq, now_ns(), &out[..n]);
+                    if conn2.send_datagram(d.into()).is_err() {
+                        break;
+                    }
+                }
+            }
+            tracing::info!("mic-test: done");
+        });
+    }
+
     // Closed-flag for the blocking receive loop.
     let closed = std::sync::Arc::new(std::sync::atomic::AtomicBool::new(false));
     {

crates/punktfunk-core/src/abi.rs

@@ -885,6 +885,42 @@ pub unsafe extern "C" fn punktfunk_connection_send_input(
     })
 }
 
+/// Send one Opus mic frame to the host as a QUIC datagram (48 kHz; the host decodes it into a
+/// virtual microphone source its apps can record). Non-blocking enqueue; the host uses `seq`/
+/// `pts_ns` (the caller's own counters) only for diagnostics. `opus_data`/`len` may be empty
+/// (a DTX silence frame). The data is copied; the caller may reuse the buffer after this returns.
+///
+/// # Safety
+/// `c` is a valid connection handle; `opus_data` is valid for `len` bytes (or `len == 0`).
+#[cfg(feature = "quic")]
+#[no_mangle]
+pub unsafe extern "C" fn punktfunk_connection_send_mic(
+    c: *mut PunktfunkConnection,
+    opus_data: *const u8,
+    len: usize,
+    seq: u32,
+    pts_ns: u64,
+) -> PunktfunkStatus {
+    guard(|| {
+        let c = match unsafe { c.as_ref() } {
+            Some(c) => c,
+            None => return PunktfunkStatus::NullPointer,
+        };
+        if opus_data.is_null() && len != 0 {
+            return PunktfunkStatus::NullPointer;
+        }
+        let opus = if len == 0 {
+            Vec::new()
+        } else {
+            unsafe { std::slice::from_raw_parts(opus_data, len) }.to_vec()
+        };
+        match c.inner.send_mic(seq, pts_ns, opus) {
+            Ok(()) => PunktfunkStatus::Ok,
+            Err(e) => e.status(),
+        }
+    })
+}
+
 /// The currently active session mode — the Welcome's, until an accepted
 /// [`punktfunk_connection_request_mode`] switches it. Safe any time after connect.
 ///

crates/punktfunk-core/src/client.rs

@@ -50,6 +50,8 @@ pub struct NativeClient {
     audio: Receiver<AudioPacket>,
     rumble: Receiver<(u16, u16, u16)>,
     input_tx: tokio::sync::mpsc::UnboundedSender<InputEvent>,
+    /// Outbound mic frames `(seq, pts_ns, opus)` → encoded as 0xCB datagrams by the worker.
+    mic_tx: tokio::sync::mpsc::UnboundedSender<(u32, u64, Vec<u8>)>,
     reconfig_tx: tokio::sync::mpsc::UnboundedSender<Mode>,
     shutdown: Arc<AtomicBool>,
     worker: Option<std::thread::JoinHandle<()>>,
@@ -85,6 +87,7 @@ impl NativeClient {
         let (audio_tx, audio_rx) = std::sync::mpsc::sync_channel::<AudioPacket>(AUDIO_QUEUE);
         let (rumble_tx, rumble_rx) = std::sync::mpsc::sync_channel::<(u16, u16, u16)>(RUMBLE_QUEUE);
         let (input_tx, input_rx) = tokio::sync::mpsc::unbounded_channel::<InputEvent>();
+        let (mic_tx, mic_rx) = tokio::sync::mpsc::unbounded_channel::<(u32, u64, Vec<u8>)>();
         let (reconfig_tx, reconfig_rx) = tokio::sync::mpsc::unbounded_channel::<Mode>();
         let (ready_tx, ready_rx) = std::sync::mpsc::channel::<Result<(Mode, [u8; 32])>>();
         let shutdown = Arc::new(AtomicBool::new(false));
@@ -118,6 +121,7 @@ impl NativeClient {
                     audio_tx,
                     rumble_tx,
                     input_rx,
+                    mic_rx,
                     reconfig_rx,
                     ready_tx,
                     shutdown: shutdown_w,
@@ -140,6 +144,7 @@ impl NativeClient {
             audio: audio_rx,
             rumble: rumble_rx,
             input_tx,
+            mic_tx,
             reconfig_tx,
             shutdown,
             worker: Some(worker),
@@ -296,6 +301,16 @@ impl NativeClient {
     pub fn send_input(&self, ev: &InputEvent) -> Result<()> {
         self.input_tx.send(*ev).map_err(|_| PunktfunkError::Closed)
     }
+
+    /// Queue one Opus mic frame for delivery as a 0xCB uplink datagram (the inverse of
+    /// [`next_audio`](Self::next_audio)). `seq`/`pts_ns` are the caller's own counters (the host
+    /// uses them only for diagnostics). The host decodes it into a virtual microphone source.
+    /// Best-effort — like every datagram, it's dropped under loss; no retransmit.
+    pub fn send_mic(&self, seq: u32, pts_ns: u64, opus: Vec<u8>) -> Result<()> {
+        self.mic_tx
+            .send((seq, pts_ns, opus))
+            .map_err(|_| PunktfunkError::Closed)
+    }
 }
 
 impl Drop for NativeClient {
@@ -318,6 +333,7 @@ struct WorkerArgs {
     audio_tx: SyncSender<AudioPacket>,
     rumble_tx: SyncSender<(u16, u16, u16)>,
     input_rx: tokio::sync::mpsc::UnboundedReceiver<InputEvent>,
+    mic_rx: tokio::sync::mpsc::UnboundedReceiver<(u32, u64, Vec<u8>)>,
     reconfig_rx: tokio::sync::mpsc::UnboundedReceiver<Mode>,
     ready_tx: std::sync::mpsc::Sender<Result<(Mode, [u8; 32])>>,
     shutdown: Arc<AtomicBool>,
@@ -338,6 +354,7 @@ async fn worker_main(args: WorkerArgs) {
         audio_tx,
         rumble_tx,
         mut input_rx,
+        mut mic_rx,
         mut reconfig_rx,
         ready_tx,
         shutdown,
@@ -429,6 +446,15 @@ async fn worker_main(args: WorkerArgs) {
         }
     });
 
+    // Mic task: embedder Opus mic frames → 0xCB uplink datagrams (best-effort, dropped on loss).
+    let mic_conn = conn.clone();
+    tokio::spawn(async move {
+        while let Some((seq, pts_ns, opus)) = mic_rx.recv().await {
+            let d = crate::quic::encode_mic_datagram(seq, pts_ns, &opus);
+            let _ = mic_conn.send_datagram(d.into());
+        }
+    });
+
     // Control task: the handshake stream stays open for mid-stream renegotiation. One
     // request at a time — write Reconfigure, await Reconfigured, publish the active mode.
     {

crates/punktfunk-core/src/quic.rs

@@ -554,10 +554,14 @@ pub fn frame(payload: &[u8]) -> Vec<u8> {
 }
 
 /// Datagram wire tags. Video rides UDP; everything low-rate rides QUIC datagrams,
-/// demultiplexed by the first byte: input = [`crate::input::INPUT_MAGIC`] (0xC8),
+/// demultiplexed by the first byte: input = [`crate::input::INPUT_MAGIC`] (0xC8, client→host),
-/// audio = [`AUDIO_MAGIC`], rumble = [`RUMBLE_MAGIC`].
+/// audio = [`AUDIO_MAGIC`] (0xC9, host→client), rumble = [`RUMBLE_MAGIC`] (0xCA, host→client),
+/// mic = [`MIC_MAGIC`] (0xCB, client→host).
 pub const AUDIO_MAGIC: u8 = 0xC9;
 pub const RUMBLE_MAGIC: u8 = 0xCA;
+/// Microphone uplink: the client's mic, Opus-encoded, client → host (the inverse of
+/// [`AUDIO_MAGIC`]). The host feeds it into a virtual PipeWire source so its apps can record it.
+pub const MIC_MAGIC: u8 = 0xCB;
 
 /// Audio datagram, host → client: `[0xC9][u32 seq LE][u64 pts_ns LE][opus payload]`.
 /// One Opus frame per datagram (5 ms — well under any MTU); QUIC already encrypts.
@@ -600,6 +604,27 @@ pub fn decode_rumble_datagram(b: &[u8]) -> Option<(u16, u16, u16)> {
     Some((u16at(1), u16at(3), u16at(5)))
 }
 
+/// Mic datagram, client → host: `[0xCB][u32 seq LE][u64 pts_ns LE][opus payload]` — the same
+/// layout as [`encode_audio_datagram`] with [`MIC_MAGIC`], one Opus frame per datagram.
+pub fn encode_mic_datagram(seq: u32, pts_ns: u64, opus: &[u8]) -> Vec<u8> {
+    let mut b = Vec::with_capacity(13 + opus.len());
+    b.push(MIC_MAGIC);
+    b.extend_from_slice(&seq.to_le_bytes());
+    b.extend_from_slice(&pts_ns.to_le_bytes());
+    b.extend_from_slice(opus);
+    b
+}
+
+/// Parse a mic datagram → `(seq, pts_ns, opus payload)`. `None` on bad tag/length.
+pub fn decode_mic_datagram(b: &[u8]) -> Option<(u32, u64, &[u8])> {
+    if b.len() < 13 || b[0] != MIC_MAGIC {
+        return None;
+    }
+    let seq = u32::from_le_bytes(b[1..5].try_into().unwrap());
+    let pts_ns = u64::from_le_bytes(b[5..13].try_into().unwrap());
+    Some((seq, pts_ns, &b[13..]))
+}
+
 /// Async framed-message IO over a quinn stream (`u16 LE length || payload`).
 pub mod io {
     /// Read one framed message (bounded at 64 KiB — control messages are tiny).
@@ -1178,6 +1203,25 @@ mod tests {
         assert!(decode_rumble_datagram(&d[..6]).is_none());
     }
 
+    #[test]
+    fn mic_datagram_roundtrip_and_disjoint_from_audio() {
+        let opus = [0x5Au8; 80];
+        let d = encode_mic_datagram(42, 9_999, &opus);
+        assert_eq!(d[0], MIC_MAGIC);
+        let (seq, pts, payload) = decode_mic_datagram(&d).unwrap();
+        assert_eq!((seq, pts), (42, 9_999));
+        assert_eq!(payload, opus);
+        assert!(decode_mic_datagram(&d[..12]).is_none()); // truncated
+                                                          // Tag separation: a mic datagram is not an audio datagram and vice-versa.
+        assert!(decode_audio_datagram(&d).is_none());
+        assert!(decode_mic_datagram(&encode_audio_datagram(1, 2, &opus)).is_none());
+        // Empty payload (DTX) is legal.
+        assert!(decode_mic_datagram(&encode_mic_datagram(0, 0, &[]))
+            .unwrap()
+            .2
+            .is_empty());
+    }
+
     #[test]
     fn fingerprint_is_sha256_of_der() {
         // Stable across calls, distinct for distinct certs.

crates/punktfunk-host/src/audio.rs

@@ -42,5 +42,32 @@ pub fn open_audio_capture(_channels: u32) -> Result<Box<dyn AudioCapturer>> {
     anyhow::bail!("audio capture requires Linux + PipeWire")
 }
 
+/// The inverse of [`AudioCapturer`]: a virtual microphone the host *produces*. It registers a
+/// PipeWire `Audio/Source` node that host apps can record from; the host [`push`](Self::push)es
+/// decoded client-mic PCM (interleaved `f32` at [`SAMPLE_RATE`]) into it, and PipeWire delivers
+/// it to whichever app records the source — silence when no input is flowing. This is how the
+/// client's microphone reaches host applications (mic passthrough).
+pub trait VirtualMic: Send {
+    /// Push one chunk of interleaved `f32` PCM. Non-blocking — drops if PipeWire is behind
+    /// (mic audio is lossy/real-time; a stale chunk is worse than a dropped one).
+    fn push(&self, pcm: &[f32]);
+
+    /// The interleaved channel count the source was opened with.
+    fn channels(&self) -> u32 {
+        CHANNELS as u32
+    }
+}
+
+/// Open a virtual microphone PipeWire source with `channels` interleaved channels (1 or 2).
+#[cfg(target_os = "linux")]
+pub fn open_virtual_mic(channels: u32) -> Result<Box<dyn VirtualMic>> {
+    linux::PwMicSource::open(channels).map(|m| Box::new(m) as Box<dyn VirtualMic>)
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn open_virtual_mic(_channels: u32) -> Result<Box<dyn VirtualMic>> {
+    anyhow::bail!("virtual mic requires Linux + PipeWire")
+}
+
 #[cfg(target_os = "linux")]
 mod linux;

crates/punktfunk-host/src/audio/linux.rs

@@ -13,8 +13,9 @@
 //! stream down promptly — required so a surround session can replace a stereo capturer
 //! without leaking a PipeWire consumer (see CLAUDE.md: a wedged link head-blocks the daemon).
 
-use super::{AudioCapturer, SAMPLE_RATE};
+use super::{AudioCapturer, VirtualMic, SAMPLE_RATE};
 use anyhow::{anyhow, Context, Result};
+use std::collections::VecDeque;
 use std::sync::mpsc::{sync_channel, Receiver, RecvTimeoutError};
 use std::thread;
 use std::time::Duration;
@@ -105,6 +106,232 @@ fn spa_positions(channels: u32) -> [u32; 64] {
     pos
 }
 
+/// Virtual microphone: a PipeWire `Audio/Source` node host apps can record from. The host pushes
+/// decoded client-mic PCM in; the loop thread's producer callback drains it (silence on
+/// underrun) into PipeWire buffers. Mirrors [`PwAudioCapturer`] but inverted (Direction::Output).
+pub struct PwMicSource {
+    pcm: std::sync::mpsc::SyncSender<Vec<f32>>,
+    channels: u32,
+    quit: pipewire::channel::Sender<Terminate>,
+}
+
+impl PwMicSource {
+    pub fn open(channels: u32) -> Result<PwMicSource> {
+        anyhow::ensure!(
+            matches!(channels, 1 | 2),
+            "virtual mic supports 1 or 2 channels, got {channels}"
+        );
+        let (pcm_tx, pcm_rx) = sync_channel::<Vec<f32>>(64);
+        let (quit_tx, quit_rx) = pipewire::channel::channel::<Terminate>();
+        thread::Builder::new()
+            .name("punktfunk-pw-mic".into())
+            .spawn(move || {
+                if let Err(e) = mic_pw_thread(pcm_rx, quit_rx, channels) {
+                    tracing::error!(error = %format!("{e:#}"), "pipewire virtual-mic thread failed");
+                }
+            })
+            .context("spawn pipewire virtual-mic thread")?;
+        Ok(PwMicSource {
+            pcm: pcm_tx,
+            channels,
+            quit: quit_tx,
+        })
+    }
+}
+
+impl Drop for PwMicSource {
+    fn drop(&mut self) {
+        let _ = self.quit.send(Terminate);
+    }
+}
+
+impl VirtualMic for PwMicSource {
+    fn push(&self, pcm: &[f32]) {
+        let _ = self.pcm.try_send(pcm.to_vec()); // drop if the PipeWire side is behind
+    }
+    fn channels(&self) -> u32 {
+        self.channels
+    }
+}
+
+/// Producer-side state for the virtual-mic loop: incoming decoded PCM and a small ring buffer
+/// the process callback drains into PipeWire buffers (capped, so latency stays bounded).
+/// `primed` is a jitter buffer gate — see the process callback.
+struct MicUserData {
+    rx: Receiver<Vec<f32>>,
+    ring: VecDeque<f32>,
+    channels: usize,
+    primed: bool,
+}
+
+fn mic_pw_thread(
+    pcm_rx: Receiver<Vec<f32>>,
+    quit_rx: pipewire::channel::Receiver<Terminate>,
+    channels: u32,
+) -> Result<()> {
+    use pipewire as pw;
+    use pw::{properties::properties, spa};
+    use spa::param::audio::{AudioFormat, AudioInfoRaw};
+    use spa::pod::Pod;
+
+    crate::pwinit::ensure_init();
+    let mainloop = pw::main_loop::MainLoopRc::new(None).context("pw mic MainLoop")?;
+    let context = pw::context::ContextRc::new(&mainloop, None).context("pw mic Context")?;
+    let core = context
+        .connect_rc(None)
+        .context("pw mic connect (is PipeWire running in this session?)")?;
+
+    let _quit_guard = quit_rx.attach(mainloop.loop_(), {
+        let mainloop = mainloop.clone();
+        move |_| mainloop.quit()
+    });
+
+    // media.class=Audio/Source advertises us as a microphone (a recordable source), NOT a
+    // playback stream — without it, Direction::Output + Playback would route to the speakers.
+    let stream = pw::stream::StreamBox::new(
+        &core,
+        "punktfunk-mic",
+        properties! {
+            *pw::keys::MEDIA_TYPE        => "Audio",
+            *pw::keys::MEDIA_CLASS       => "Audio/Source",
+            *pw::keys::NODE_NAME         => "punktfunk-mic",
+            *pw::keys::NODE_DESCRIPTION  => "Punktfunk Remote Microphone",
+            // ~5 ms quantum (one Opus frame) so recording apps get smooth low-latency chunks.
+            *pw::keys::NODE_LATENCY      => "240/48000",
+        },
+    )
+    .context("pw mic Stream")?;
+
+    let ud = MicUserData {
+        rx: pcm_rx,
+        ring: VecDeque::new(),
+        channels: channels as usize,
+        primed: false,
+    };
+
+    let _listener = stream
+        .add_local_listener_with_user_data(ud)
+        .state_changed(|_s, _ud, old, new| {
+            tracing::info!(?old, ?new, "pipewire virtual-mic stream state");
+        })
+        .param_changed(|_s, _ud, id, param| {
+            let Some(param) = param else { return };
+            if id != pw::spa::param::ParamType::Format.as_raw() {
+                return;
+            }
+            let mut info = AudioInfoRaw::default();
+            if info.parse(param).is_ok() {
+                tracing::info!(
+                    format = ?info.format(),
+                    rate = info.rate(),
+                    channels = info.channels(),
+                    "virtual-mic format negotiated"
+                );
+            }
+        })
+        .process(|stream, ud| {
+            let outcome = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+                let Some(mut buffer) = stream.dequeue_buffer() else {
+                    return;
+                };
+                // Pull all newly-decoded PCM into the ring.
+                while let Ok(frame) = ud.rx.try_recv() {
+                    ud.ring.extend(frame);
+                }
+                let stride = 4 * ud.channels; // F32LE interleaved
+                let datas = buffer.datas_mut();
+                if datas.is_empty() {
+                    return;
+                }
+                let data = &mut datas[0];
+                let want_frames = data.data().map(|s| s.len() / stride).unwrap_or(0);
+                let want = want_frames * ud.channels; // interleaved samples this quantum needs
+                static FIRST: std::sync::atomic::AtomicBool =
+                    std::sync::atomic::AtomicBool::new(true);
+                if FIRST.swap(false, std::sync::atomic::Ordering::Relaxed) {
+                    tracing::info!(
+                        quantum_frames = want_frames,
+                        quantum_ms = want_frames as f32 / 48.0,
+                        "virtual-mic consumer connected"
+                    );
+                }
+
+                // Adaptive jitter buffer. The client pushes 5 ms frames; the recorder pulls a
+                // whole *quantum* (often 20–43 ms) from an independent clock. A drain of one
+                // quantum must not outrun what's buffered, or every call underruns to silence
+                // (the original ~58% gaps). So prime to ~3 quanta before producing, hold there,
+                // and re-prime only after a genuine full drain (the client went quiet). The ring
+                // is capped at a few quanta so latency stays bounded.
+                let target = (3 * want).clamp(720 * ud.channels, 9600 * ud.channels);
+                while ud.ring.len() > target.max(want) + want {
+                    ud.ring.pop_front(); // bound latency: drop the oldest beyond ~1 quantum slack
+                }
+                if !ud.primed && ud.ring.len() >= target {
+                    ud.primed = true;
+                }
+
+                let n_frames = if let Some(slice) = data.data() {
+                    for k in 0..want {
+                        let s = if ud.primed {
+                            ud.ring.pop_front().unwrap_or(0.0) // silence on a momentary underrun
+                        } else {
+                            0.0 // not yet primed — emit silence while the buffer fills
+                        };
+                        let off = k * 4;
+                        slice[off..off + 4].copy_from_slice(&s.to_le_bytes());
+                    }
+                    want_frames
+                } else {
+                    0
+                };
+                if ud.ring.is_empty() {
+                    ud.primed = false; // fully drained — re-prime before producing again
+                }
+                let chunk = data.chunk_mut();
+                *chunk.offset_mut() = 0;
+                *chunk.stride_mut() = stride as _;
+                *chunk.size_mut() = (stride * n_frames) as _;
+            }));
+            if outcome.is_err() {
+                tracing::error!("panic in pipewire virtual-mic callback");
+            }
+        })
+        .register()
+        .context("register virtual-mic stream listener")?;
+
+    let mut info = AudioInfoRaw::new();
+    info.set_format(AudioFormat::F32LE);
+    info.set_rate(SAMPLE_RATE);
+    info.set_channels(channels);
+    info.set_position(spa_positions(channels));
+    let obj = pw::spa::pod::Object {
+        type_: pw::spa::utils::SpaTypes::ObjectParamFormat.as_raw(),
+        id: pw::spa::param::ParamType::EnumFormat.as_raw(),
+        properties: info.into(),
+    };
+    let values: Vec<u8> = pw::spa::pod::serialize::PodSerializer::serialize(
+        std::io::Cursor::new(Vec::new()),
+        &pw::spa::pod::Value::Object(obj),
+    )
+    .context("serialize mic format pod")?
+    .0
+    .into_inner();
+    let mut params = [Pod::from_bytes(&values).context("mic pod from bytes")?];
+
+    stream
+        .connect(
+            spa::utils::Direction::Output, // we PRODUCE samples (a source)
+            None,
+            pw::stream::StreamFlags::AUTOCONNECT | pw::stream::StreamFlags::MAP_BUFFERS,
+            &mut params,
+        )
+        .context("pw mic stream connect")?;
+
+    mainloop.run();
+    tracing::debug!("pipewire virtual-mic loop exited (source dropped)");
+    Ok(())
+}
+
 fn pw_thread(
     tx: std::sync::mpsc::SyncSender<Vec<f32>>,
     quit_rx: pipewire::channel::Receiver<Terminate>,

crates/punktfunk-host/src/m3.rs

@@ -185,6 +185,9 @@ async fn serve(opts: M3Options) -> Result<()> {
     // session — which, under rapid client reconnects, raced a prior session's portal teardown and
     // wedged KWin's EIS setup ("EIS setup timed out"). Gamepads stay per-session (uinput).
     let injector = InjectorService::start();
+    // One virtual microphone for the whole host lifetime (see MicService): the client's mic uplink
+    // (0xCB) is Opus-decoded and fed into a persistent PipeWire Audio/Source host apps record from.
+    let mic_service = MicService::start();
     let paired_at = match &opts.paired_store {
         Some(p) => p.clone(),
         None => paired_path()?,
@@ -233,6 +236,7 @@ async fn serve(opts: M3Options) -> Result<()> {
             &opts,
             &audio_cap,
             injector.sender(),
+            mic_service.sender(),
             &fingerprint,
             &paired,
             &last_pairing,
@@ -350,6 +354,7 @@ async fn serve_session(
     opts: &M3Options,
     audio_cap: &AudioCapSlot,
     inj_tx: std::sync::mpsc::Sender<InputEvent>,
+    mic_tx: std::sync::mpsc::Sender<Vec<u8>>,
     host_fp: &[u8; 32],
     paired: &PairedStore,
     last_pairing: &std::sync::Mutex<Option<std::time::Instant>>,
@@ -521,18 +526,30 @@ async fn serve_session(
             .spawn(move || input_thread(input_rx, conn, inj_tx))
             .context("spawn input thread")?
     };
+    // One reader for ALL client→host datagrams, demuxed by magic byte (two read_datagram loops
+    // would race for datagrams): 0xCB → mic uplink (Opus, forwarded to the host-lifetime mic
+    // service), 0xC8 → input (forwarded to the per-session input thread). The magics are disjoint,
+    // so decode order doesn't matter. Unknown tags are ignored.
     let input_conn = conn.clone();
     tokio::spawn(async move {
-        let mut count = 0u64;
+        let (mut input_count, mut mic_count) = (0u64, 0u64);
         while let Ok(d) = input_conn.read_datagram().await {
-            if let Some(ev) = InputEvent::decode(&d) {
+            if let Some((_seq, _pts, opus)) = punktfunk_core::quic::decode_mic_datagram(&d) {
-                count += 1;
+                mic_count += 1;
+                // Host-lifetime mic service; a send error just means the host is shutting down.
+                let _ = mic_tx.send(opus.to_vec());
+            } else if let Some(ev) = InputEvent::decode(&d) {
+                input_count += 1;
                 if input_tx.send(ev).is_err() {
                     break;
                 }
             }
         }
-        tracing::info!(count, "input datagram stream ended");
+        tracing::info!(
+            input = input_count,
+            mic = mic_count,
+            "client datagram stream ended"
+        );
     });
 
     // Stop signal: stream duration elapsed or the client went away.
@@ -758,6 +775,92 @@ fn injector_service_thread(rx: std::sync::mpsc::Receiver<InputEvent>) {
     tracing::debug!("injector service stopped (host shutting down)");
 }
 
+/// Mic is 48 kHz stereo — matches the Opus stereo decoder and the host→client audio layout.
+const MIC_CHANNELS: u32 = 2;
+
+/// Host-lifetime virtual microphone, shared across punktfunk/1 sessions (mirror of
+/// [`InjectorService`]). One thread owns the PipeWire `Audio/Source` + an Opus decoder; sessions
+/// forward the client's Opus mic frames over a clonable `Send` channel, the thread decodes and
+/// feeds the source. Opened lazily on the first frame, the source node persists across sessions
+/// (no per-session registration churn), and reopens after a backoff if the source/decoder fails.
+struct MicService {
+    tx: std::sync::mpsc::Sender<Vec<u8>>,
+}
+
+impl MicService {
+    fn start() -> MicService {
+        let (tx, rx) = std::sync::mpsc::channel::<Vec<u8>>();
+        if let Err(e) = std::thread::Builder::new()
+            .name("punktfunk-m3-mic".into())
+            .spawn(move || mic_service_thread(rx))
+        {
+            tracing::error!(error = %e, "mic service thread spawn failed — mic passthrough disabled");
+        }
+        MicService { tx }
+    }
+
+    /// A sender a session forwards the client's Opus mic frames to. Cloned per session; dropping a
+    /// clone does NOT stop the service (it holds the original sender for the host life).
+    fn sender(&self) -> std::sync::mpsc::Sender<Vec<u8>> {
+        self.tx.clone()
+    }
+}
+
+/// The host-lifetime mic worker: lazily open the virtual mic + decoder, then Opus-decode each
+/// forwarded frame and push the PCM into the source. Reopen (after [`INJECTOR_REOPEN_BACKOFF`])
+/// on open failure or a decode error. Exits when every session sender and the service's own
+/// sender drop (host shutdown), tearing the PipeWire source down.
+fn mic_service_thread(rx: std::sync::mpsc::Receiver<Vec<u8>>) {
+    let mut mic: Option<Box<dyn crate::audio::VirtualMic>> = None;
+    let mut decoder: Option<opus::Decoder> = None;
+    let mut last_failed: Option<std::time::Instant> = None;
+    let mut pcm = vec![0f32; 5760 * MIC_CHANNELS as usize]; // up to 120 ms scratch
+    for opus_frame in rx {
+        if opus_frame.is_empty() {
+            continue; // DTX silence — the source underruns to silence on its own
+        }
+        if mic.is_none() || decoder.is_none() {
+            if last_failed.is_some_and(|t| t.elapsed() < INJECTOR_REOPEN_BACKOFF) {
+                continue; // still within the reopen backoff window
+            }
+            let opened = crate::audio::open_virtual_mic(MIC_CHANNELS).and_then(|m| {
+                let d = opus::Decoder::new(48_000, opus::Channels::Stereo)
+                    .map_err(|e| anyhow!("opus decoder: {e}"))?;
+                Ok((m, d))
+            });
+            match opened {
+                Ok((m, d)) => {
+                    tracing::info!("punktfunk/1 virtual mic ready (host-lifetime)");
+                    mic = Some(m);
+                    decoder = Some(d);
+                    last_failed = None;
+                }
+                Err(e) => {
+                    tracing::error!(error = %format!("{e:#}"), "virtual mic unavailable — will retry");
+                    last_failed = Some(std::time::Instant::now());
+                    continue;
+                }
+            }
+        }
+        let (Some(m), Some(dec)) = (mic.as_ref(), decoder.as_mut()) else {
+            continue;
+        };
+        match dec.decode_float(&opus_frame, &mut pcm, false) {
+            Ok(samples_per_ch) => {
+                let total = (samples_per_ch * MIC_CHANNELS as usize).min(pcm.len());
+                m.push(&pcm[..total]);
+            }
+            Err(e) => {
+                tracing::warn!(error = %e, "mic opus decode failed — reopening");
+                mic = None;
+                decoder = None;
+                last_failed = Some(std::time::Instant::now());
+            }
+        }
+    }
+    tracing::debug!("mic service stopped (host shutting down)");
+}
+
 /// The per-session input thread: route pointer/keyboard events to the host-lifetime injector
 /// service (`inj_tx`) and gamepad events to this session's own [`GamepadManager`]
 /// (crate::inject::gamepad), with force feedback pumped between events and sent back as rumble

include/punktfunk_core.h

@@ -135,8 +135,9 @@
 
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Datagram wire tags. Video rides UDP; everything low-rate rides QUIC datagrams,
-// demultiplexed by the first byte: input = [`crate::input::INPUT_MAGIC`] (0xC8),
+// demultiplexed by the first byte: input = [`crate::input::INPUT_MAGIC`] (0xC8, client→host),
-// audio = [`AUDIO_MAGIC`], rumble = [`RUMBLE_MAGIC`].
+// audio = [`AUDIO_MAGIC`] (0xC9, host→client), rumble = [`RUMBLE_MAGIC`] (0xCA, host→client),
+// mic = [`MIC_MAGIC`] (0xCB, client→host).
 #define PUNKTFUNK_AUDIO_MAGIC 201
 #endif
 
@@ -144,6 +145,12 @@
 #define PUNKTFUNK_RUMBLE_MAGIC 202
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Microphone uplink: the client's mic, Opus-encoded, client → host (the inverse of
+// [`AUDIO_MAGIC`]). The host feeds it into a virtual PipeWire source so its apps can record it.
+#define MIC_MAGIC 203
+#endif
+
 // Stable C ABI status codes. `Ok` is 0; all errors are negative so callers can
 // test `rc < 0`. Do not renumber existing variants — only append.
 enum PunktfunkStatus
@@ -507,6 +514,21 @@ PunktfunkStatus punktfunk_connection_send_input(PunktfunkConnection *c,
                                                 const PunktfunkInputEvent *ev);
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Send one Opus mic frame to the host as a QUIC datagram (48 kHz; the host decodes it into a
+// virtual microphone source its apps can record). Non-blocking enqueue; the host uses `seq`/
+// `pts_ns` (the caller's own counters) only for diagnostics. `opus_data`/`len` may be empty
+// (a DTX silence frame). The data is copied; the caller may reuse the buffer after this returns.
+//
+// # Safety
+// `c` is a valid connection handle; `opus_data` is valid for `len` bytes (or `len == 0`).
+PunktfunkStatus punktfunk_connection_send_mic(PunktfunkConnection *c,
+                                              const uint8_t *opus_data,
+                                              uintptr_t len,
+                                              uint32_t seq,
+                                              uint64_t pts_ns);
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // The currently active session mode — the Welcome's, until an accepted
 // [`punktfunk_connection_request_mode`] switches it. Safe any time after connect.