Restart microphone capture on audio device change

When a capture device disappears mid-call (e.g. unplugging a Bluetooth
headset), the local microphone went silent and never recovered:
MicrophoneSource stayed bound to the gone device name and never
re-registered the AudioProbe tap that Unity detaches when it rebuilds
its audio graph.

Subscribe to AudioSettings.OnAudioConfigurationChanged (mirroring the
playback-side AudioStream handler) and restart capture on a device
change, resolving to the OS default device when the preferred device is
no longer present. Track the active device separately from the preferred
one so Microphone.IsRecording/GetPosition/End target the right device,
and guard against overlapping restarts.

The native source's rate is fixed at construction, so if the device
change moves Unity's DSP output rate, frames are still dropped; warn
clearly in that case. Full rate-change recovery follows separately.

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

Author: MaxHeimbrock

Runtime/Scripts/MicrophoneSource.cs

@@ -14,12 +14,20 @@ namespace LiveKit
     sealed public class MicrophoneSource : RtcAudioSource
     {
         private readonly GameObject _sourceObject;
+
+        // The device requested by the caller. Empty/null means "follow the OS default".
         private readonly string _deviceName;
 
+        // The device the microphone is actually recording from right now. This can differ from
+        // _deviceName when the preferred device is unavailable and we fall back to the OS default,
+        // so all Microphone.* calls (IsRecording/GetPosition/End) must use this name.
+        private string _activeDeviceName;
+
         public override event Action<float[], int, int> AudioRead;
 
         private bool _disposed = false;
         private bool _started = false;
+        private bool _restarting = false;
 
         /// <summary>
         /// Creates a new microphone source for the given device.
@@ -54,6 +62,10 @@ public override void Start()
                 throw new InvalidOperationException("Microphone access not authorized");
 
             MonoBehaviourContext.OnApplicationPauseEvent += OnApplicationPause;
+            // Restart capture when the system audio device changes (e.g. a Bluetooth headset is
+            // unplugged). Unity rebuilds its audio graph on a device change, which both detaches
+            // the AudioProbe tap and leaves Microphone.Start bound to a now-gone device.
+            AudioSettings.OnAudioConfigurationChanged += OnAudioConfigurationChanged;
             MonoBehaviourContext.RunCoroutine(StartMicrophone());
 
             _started = true;
@@ -75,11 +87,16 @@ private IEnumerator StartMicrophone()
                 yield break;
             }
 
+            // Resolve which device to record from. Falls back to the OS default when the
+            // preferred device is gone, so an unplugged headset transparently hands off to the
+            // built-in microphone.
+            _activeDeviceName = ResolveCaptureDevice();
+
             AudioClip clip = null;
             try
             {
                 clip = Microphone.Start(
-                    _deviceName,
+                    _activeDeviceName,
                     loop: true,
                     lengthSec: 1,
                     frequency: (int)_expectedSampleRate
@@ -123,20 +140,20 @@ private IEnumerator StartMicrophone()
             // Wait for microphone to actually start producing data with a timeout
             const float timeout = 2f;
             float elapsed = 0f;
-            while (Microphone.GetPosition(_deviceName) <= 0 && elapsed < timeout)
+            while (Microphone.GetPosition(_activeDeviceName) <= 0 && elapsed < timeout)
             {
                 yield return new WaitForSeconds(0.05f);
                 elapsed += 0.05f;
             }
 
-            if (Microphone.GetPosition(_deviceName) <= 0)
+            if (Microphone.GetPosition(_activeDeviceName) <= 0)
             {
                 Utils.Error($"MicrophoneSource: Microphone did not start producing data after {timeout}s");
                 yield break;
             }
 
             source.Play();
-            Utils.Debug($"MicrophoneSource device='{_deviceName}' started successfully");
+            Utils.Debug($"MicrophoneSource device='{_activeDeviceName ?? "<default>"}' started successfully");
         }
 
         /// <summary>
@@ -147,13 +164,14 @@ public override void Stop()
             base.Stop();
             MonoBehaviourContext.RunCoroutine(StopMicrophone());
             MonoBehaviourContext.OnApplicationPauseEvent -= OnApplicationPause;
+            AudioSettings.OnAudioConfigurationChanged -= OnAudioConfigurationChanged;
             _started = false;
         }
 
         private IEnumerator StopMicrophone()
         {
-            if (Microphone.IsRecording(_deviceName))
-                Microphone.End(_deviceName);
+            if (Microphone.IsRecording(_activeDeviceName))
+                Microphone.End(_activeDeviceName);
 
             // Check if GameObject is still valid before trying to access components
             if (_sourceObject != null)
@@ -170,7 +188,7 @@ private IEnumerator StopMicrophone()
                     UnityEngine.Object.Destroy(source);
             }
 
-            Utils.Debug($"MicrophoneSource device='{_deviceName}' stopped");
+            Utils.Debug($"MicrophoneSource device='{_activeDeviceName ?? "<default>"}' stopped");
             yield return null;
         }
 
@@ -197,8 +215,54 @@ private void OnApplicationPause(bool pause)
             }
         }
 
+        // Picks the device name to pass to Microphone.Start. An empty preferred name, or a
+        // preferred device that is no longer connected, resolves to null so Unity records from
+        // the current OS default device.
+        private string ResolveCaptureDevice()
+        {
+            if (string.IsNullOrEmpty(_deviceName))
+                return null;
+
+            if (Array.IndexOf(Microphone.devices, _deviceName) >= 0)
+                return _deviceName;
+
+            Utils.Debug($"MicrophoneSource: preferred device '{_deviceName}' is no longer available, falling back to the OS default");
+            return null;
+        }
+
+        // Fires on the main thread when Unity's audio configuration changes, including when the
+        // system capture/playback device changes (e.g. unplugging a Bluetooth headset). Mirrors
+        // AudioStream.OnAudioConfigurationChanged on the playback side.
+        private void OnAudioConfigurationChanged(bool deviceWasChanged)
+        {
+            if (!_started)
+                return;
+
+            // The native source's rate is fixed at construction and RtcAudioSource drops frames
+            // whose rate doesn't match it. If the device change moved Unity's DSP output rate,
+            // restarting capture alone won't recover audio — warn so the silence is diagnosable.
+            // Full recovery (recreating the native source at the new rate) is handled separately.
+            var outputSampleRate = (uint)AudioSettings.outputSampleRate;
+            if (outputSampleRate != _expectedSampleRate)
+            {
+                Utils.Warning($"MicrophoneSource: audio device change moved the DSP output rate to {outputSampleRate}Hz, but the native source is fixed at {_expectedSampleRate}Hz. Captured frames will be dropped until the track is recreated at the new rate.");
+            }
+
+            if (deviceWasChanged)
+            {
+                Utils.Debug("MicrophoneSource: audio device changed, restarting capture on the current default device");
+                MonoBehaviourContext.RunCoroutine(RestartMicrophone());
+            }
+        }
+
         private IEnumerator RestartMicrophone()
         {
+            // The device-change event can fire several times around a single hardware swap;
+            // ignore re-entrant restarts so overlapping Stop/Start coroutines don't race.
+            if (_restarting)
+                yield break;
+            _restarting = true;
+
             yield return StopMicrophone();
 
             // Wait for iOS audio session to be ready before attempting to restart.
@@ -207,6 +271,8 @@ private IEnumerator RestartMicrophone()
             yield return WaitForMicrophoneReady();
 
             yield return StartMicrophone();
+
+            _restarting = false;
         }
 
         private IEnumerator WaitForMicrophoneReady()