fix: recover from microphone disconnects

MeetingAssistant.Tests/AudioMixingTests.cs

@@ -102,6 +102,23 @@ public sealed class AudioMixingTests
         Assert.Equal(2_000, BitConverter.ToInt16(chunks[0].Pcm));
     }
 
+    [Fact]
+    public async Task CompositeAudioSourceKeepsSystemAudioWhileMicrophoneIsRecovering()
+    {
+        var microphone = new WaitingAudioSource();
+        var system = new FixedAudioSource(Pcm16(10_000));
+        var source = CreateSource(microphone, system);
+        using var cancellation = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        await using var chunks = source
+            .CaptureAsync(new MeetingAssistantOptions(), cancellation.Token)
+            .GetAsyncEnumerator(cancellation.Token);
+
+        Assert.True(await chunks.MoveNextAsync());
+        Assert.Equal(10_000, BitConverter.ToInt16(chunks.Current.Pcm));
+
+        await cancellation.CancelAsync();
+    }
+
     [Fact]
     public void AdaptiveEchoCancellerReducesEchoFromMicrophoneSignal()
     {
@@ -221,6 +238,16 @@ public sealed class AudioMixingTests
         }
     }
 
+    private sealed class WaitingAudioSource : IMeetingAudioSource
+    {
+        public async IAsyncEnumerable<AudioChunk> CaptureAsync(
+            [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken)
+        {
+            await Task.Delay(Timeout.InfiniteTimeSpan, cancellationToken);
+            yield break;
+        }
+    }
+
     private sealed class OptionsCapturingAudioSource : IMeetingAudioSource
     {
         private readonly AudioChunk chunk;