fix: RNNoise mono-left-only audio via explicit ChannelMerger stereo upmix

RNNoise worklet outputs mono (1ch) which played only in the left ear.
Replace unreliable automatic up-mixing (channelCount/channelCountMode on
inputGain) with a ChannelMergerNode that duplicates the mono signal to
both L and R channels — guaranteed stereo by the Web Audio spec.

packages/web/src/audio/AudioManager.js

@@ -25,6 +25,7 @@ export class AudioManager {
     streamGeneration = 0;
     inputSwitchChain = Promise.resolve(null);
     rnnoiseNode = null;
+    stereoMerger = null;
     rnnoiseEnabled = false;
     rnnoiseReady = false;
     constructor() { }
@@ -40,11 +41,6 @@ export class AudioManager {
         const AudioContextClass = window.AudioContext || window.webkitAudioContext;
         this.ctx = new AudioContextClass({ sampleRate: 48000 });
         this.inputGain = this.ctx.createGain();
-        // Force stereo up-mix so mono sources (e.g. RNNoise worklet output)
-        // are duplicated to both L+R channels instead of left-only.
-        this.inputGain.channelCount = 2;
-        this.inputGain.channelCountMode = 'explicit';
-        this.inputGain.channelInterpretation = 'speakers';
         this.inputDestination = this.ctx.createMediaStreamDestination();
         this.analyser = this.ctx.createAnalyser();
         this.analyser.fftSize = 256;
@@ -210,9 +206,18 @@ export class AudioManager {
                     wasmBinary,
                     maxChannels: 1,
                 });
-                this.rnnoiseNode.connect(this.inputGain);
+
+                // Explicit mono→stereo: RNNoise outputs 1 channel, so duplicate it
+                // to both L and R via a ChannelMergerNode. This is spec-guaranteed
+                // stereo, unlike relying on automatic up-mixing which fails in some
+                // browsers when the source is an AudioWorkletNode.
+                this.stereoMerger = this.ctx.createChannelMerger(2);
+                this.rnnoiseNode.connect(this.stereoMerger, 0, 0); // mono → left
+                this.rnnoiseNode.connect(this.stereoMerger, 0, 1); // mono → right
+                this.stereoMerger.connect(this.inputGain);
+
                 this.rnnoiseReady = true;
-                console.log('[AudioManager] RNNoise worklet loaded and connected');
+                console.log('[AudioManager] RNNoise worklet loaded and connected (mono→stereo via ChannelMerger)');
             }
             catch (err) {
                 console.error('[AudioManager] Failed to load RNNoise worklet:', err);

packages/web/src/audio/AudioManager.ts

@@ -27,6 +27,7 @@ export class AudioManager {
   private streamGeneration = 0;
   private inputSwitchChain: Promise<MediaStream | null> = Promise.resolve(null);
   private rnnoiseNode: AudioWorkletNode | null = null;
+  private stereoMerger: ChannelMergerNode | null = null;
   private rnnoiseEnabled = false;
   private rnnoiseReady = false;
 
@@ -45,11 +46,6 @@ export class AudioManager {
     this.ctx = new AudioContextClass({ sampleRate: 48000 });
     
     this.inputGain = this.ctx.createGain();
-    // Force stereo up-mix so mono sources (e.g. RNNoise worklet output)
-    // are duplicated to both L+R channels instead of left-only.
-    this.inputGain.channelCount = 2;
-    this.inputGain.channelCountMode = 'explicit';
-    this.inputGain.channelInterpretation = 'speakers';
     this.inputDestination = this.ctx.createMediaStreamDestination();
     this.analyser = this.ctx.createAnalyser();
     this.analyser.fftSize = 256;
@@ -258,9 +254,18 @@ export class AudioManager {
           wasmBinary,
           maxChannels: 1,
         });
-        this.rnnoiseNode.connect(this.inputGain!);
+
+        // Explicit mono→stereo: RNNoise outputs 1 channel, so duplicate it
+        // to both L and R via a ChannelMergerNode. This is spec-guaranteed
+        // stereo, unlike relying on automatic up-mixing which fails in some
+        // browsers when the source is an AudioWorkletNode.
+        this.stereoMerger = this.ctx!.createChannelMerger(2);
+        this.rnnoiseNode.connect(this.stereoMerger, 0, 0); // mono → left
+        this.rnnoiseNode.connect(this.stereoMerger, 0, 1); // mono → right
+        this.stereoMerger.connect(this.inputGain!);
+
         this.rnnoiseReady = true;
-        console.log('[AudioManager] RNNoise worklet loaded and connected');
+        console.log('[AudioManager] RNNoise worklet loaded and connected (mono→stereo via ChannelMerger)');
       } catch (err) {
         console.error('[AudioManager] Failed to load RNNoise worklet:', err);
         this.rnnoiseReady = false;