Mute the Windows output while streaming its audio and sync its volume

README.md

@@ -61,7 +61,7 @@ Working today, over USB with ADB:
 | Locally drawn cursor | ✅ | ✅ | 🧪 | 🧪 | 🧪 |
 | Zero-copy hardware encode | ✅ | ✅ | 🧪 | ❌ shared-memory capture | ❌ shared-memory capture |
 | Touch, pen pressure, tilt and hover | ✅ | ✅ | 🧪 | 🧪 | 🧪 |
-| PC audio on the tablet, with volume kept in sync | 🧪 audio only, no volume sync | ✅ | 🧪 | 🧪 | 🧪 |
+| PC audio on the tablet, with volume kept in sync | 🧪 | ✅ | 🧪 | 🧪 | 🧪 |
 | Tray app with settings and live statistics | ✅ | ✅ | 🧪 | 🧪 | 🧪 |
 | Installer / packages | ✅ `.exe` | ✅ AppImage, `.deb`, Arch | 🧪 AppImage, `.deb`, Arch | 🧪 AppImage, `.deb`, Arch | 🧪 AppImage, `.deb`, Arch |
 
@@ -103,8 +103,12 @@ The PC's sound plays on the tablet while a session runs.
 - **Linux:** the host adds a PipeWire output named `ezdisplay` and makes it the default for as long
   as the session lasts, then puts the previous default back. Its volume and mute follow the tablet's
   and the other way round.
-- **Windows:** the host captures what the default output plays (WASAPI loopback). The PC keeps
-  playing it too, and the volume is not synced.
+- **Windows:** the host captures what the default output plays (WASAPI loopback) and mutes that
+  output for as long as the session lasts, so the PC stays quiet; the capture is unaffected by the
+  mute and the volume slider. The slider follows the tablet's volume and the other way round; the
+  tablet's mute stays on the tablet. Afterwards the output gets its volume back and is unmuted.
+  Unmute it with the speaker button to hear the PC as well. If the host crashes, it restores the
+  output on its next start.
 - Over a fast link the audio is uncompressed PCM, otherwise Opus. Choose with `--audio
   auto|pcm|opus|off` or `audio = "..."` in `config.toml`, or in the tray settings.
 - The tablet buffers 20 to 80 ms of audio, growing the buffer when playback stutters and shrinking

crates/ezd-host-service/src/platform.rs

@@ -47,6 +47,12 @@ pub fn audio_source() -> Option<Box<dyn AudioSource>> {
     }
 }
 
+/// Puts back what a crashed session left changed in the PC audio output (Windows mutes it).
+pub fn recover_audio_after_crash() {
+    #[cfg(windows)]
+    imp::recover_audio_after_crash();
+}
+
 /// Removes a virtual display left on the desktop for a quick reconnect (Windows).
 pub fn release_virtual_display() {
     #[cfg(windows)]

crates/ezd-host-service/src/service.rs

@@ -241,6 +241,7 @@ impl HostService {
             TcpListener::from_std(std_listener).map_err(bind_err)?
         };
         let local = listener.local_addr().map_err(bind_err)?;
+        crate::platform::recover_audio_after_crash();
 
         #[cfg(feature = "wifi")]
         let wifi = match &cfg.wifi {

crates/ezd-host-windows/Cargo.toml

@@ -36,6 +36,7 @@ windows = { version = "0.62.2", features = [
     "Win32_Graphics_Dxgi_Common",
     "Win32_Graphics_Gdi",
     "Win32_Media_Audio",
+    "Win32_Media_Audio_Endpoints",
     "Win32_Devices_Display",
     "Win32_Storage_FileSystem",
     "Win32_System_Com",

crates/ezd-host-windows/src/audio_math.rs

@@ -1,6 +1,6 @@
 //! Pure logic of the WASAPI loopback capture (`win/audio.rs`): the shared-mode format request,
-//! frame/sample math, the silent-packet flag and the packet timestamp. Platform independent and
-//! unit-tested everywhere.
+//! frame/sample math, the silent-packet flag, the packet timestamp and the saved state of the
+//! output device. Platform independent and unit-tested everywhere.
 
 /// Sample rate of the audio stream (fixed by the protocol).
 pub const SAMPLE_RATE: u32 = 48_000;
@@ -83,6 +83,56 @@ pub fn packet_samples(raw: &[u8], frames: u32, channels: u16, flags: u32) -> Vec
         .collect()
 }
 
+/// What a session changed on the output device, persisted while it runs so a host that crashed
+/// can put it back on its next start.
+#[derive(Debug, Clone, PartialEq)]
+pub struct SavedOutput {
+    /// `IMMDevice::GetId` of the output device.
+    pub device_id: String,
+    /// Master volume (the slider value, 0..=1) before the session.
+    pub volume: f32,
+    /// True if the session muted the device (it was not muted before).
+    pub muted_by_us: bool,
+}
+
+impl SavedOutput {
+    const VERSION: &str = "v1";
+
+    /// `v1;<muted_by_us 0|1>;<volume>;<device id>`.
+    pub fn encode(&self) -> String {
+        format!(
+            "{};{};{};{}",
+            Self::VERSION,
+            u8::from(self.muted_by_us),
+            self.volume,
+            self.device_id
+        )
+    }
+
+    /// Inverse of [`encode`](Self::encode); `None` for anything malformed.
+    pub fn decode(s: &str) -> Option<Self> {
+        let mut parts = s.splitn(4, ';');
+        if parts.next()? != Self::VERSION {
+            return None;
+        }
+        let muted_by_us = match parts.next()? {
+            "0" => false,
+            "1" => true,
+            _ => return None,
+        };
+        let volume: f32 = parts.next()?.parse().ok()?;
+        let device_id = parts.next()?;
+        if !(0.0..=1.0).contains(&volume) || device_id.is_empty() {
+            return None;
+        }
+        Some(Self {
+            device_id: device_id.to_owned(),
+            volume,
+            muted_by_us,
+        })
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -139,4 +189,37 @@ mod tests {
         assert_eq!(packet_samples(&raw, 1, 2, 0), vec![0.5, -0.25]);
         assert_eq!(packet_samples(&raw[..6], 2, 2, 0), vec![0.5]);
     }
+
+    #[test]
+    fn saved_output_round_trips() {
+        let saved = SavedOutput {
+            device_id: "{0.0.0.00000000}.{6a1f2c3d-0000-4e5f-8a9b-0123456789ab}".into(),
+            volume: 0.42,
+            muted_by_us: true,
+        };
+        assert_eq!(SavedOutput::decode(&saved.encode()), Some(saved.clone()));
+        let unmuted = SavedOutput {
+            muted_by_us: false,
+            volume: 1.0,
+            ..saved
+        };
+        assert_eq!(SavedOutput::decode(&unmuted.encode()), Some(unmuted));
+    }
+
+    #[test]
+    fn malformed_saved_output_is_rejected() {
+        for s in [
+            "",
+            "v1",
+            "v2;1;0.5;{dev}",
+            "v1;2;0.5;{dev}",
+            "v1;1;loud;{dev}",
+            "v1;1;1.5;{dev}",
+            "v1;1;NaN;{dev}",
+            "v1;1;0.5;",
+            "v1;1;0.5",
+        ] {
+            assert_eq!(SavedOutput::decode(s), None, "{s:?}");
+        }
+    }
 }

crates/ezd-host-windows/src/lib.rs

@@ -61,8 +61,8 @@ pub fn input_backend() -> Option<Arc<dyn InputBackend>> {
 }
 
 /// System audio capture (WASAPI loopback of the default output device); `None` on other
-/// targets. Creating it does no device work: that starts in `AudioSource::start`. The PC keeps
-/// playing the audio locally.
+/// targets. Creating it does no device work: that starts in `AudioSource::start`, which also
+/// mutes the PC output until capture stops.
 pub fn audio_source() -> Option<Box<dyn AudioSource>> {
     #[cfg(windows)]
     {
@@ -74,6 +74,13 @@ pub fn audio_source() -> Option<Box<dyn AudioSource>> {
     }
 }
 
+/// Puts back the PC audio output if a host that crashed during a session left it muted. Call
+/// once when the host starts serving. No-op on other targets.
+pub fn recover_audio_after_crash() {
+    #[cfg(windows)]
+    win::audio::recover_after_crash();
+}
+
 /// Platform-specific diagnostics for `ezd-host diag`.
 pub fn diag_section() -> serde_json::Value {
     #[cfg(windows)]