When an audio track is disabled, still pass the data to webrtc for audio processing.

content/renderer/media/webrtc_local_audio_track.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/media/webrtc_local_audio_track.h"
 
 #include "content/renderer/media/webaudio_capturer_source.h"
 #include "content/renderer/media/webrtc_audio_capturer.h"
 #include "content/renderer/media/webrtc_audio_capturer_sink_owner.h"
 #include "content/renderer/media/webrtc_local_audio_source_provider.h"
@@ skipping 136 matching lines @@
                                     int audio_delay_milliseconds,
                                     int volume,
                                     bool key_pressed) {
   scoped_refptr<WebRtcAudioCapturer> capturer;
   std::vector<int> voe_channels;
   int sample_rate = 0;
   int number_of_channels = 0;
   int number_of_frames = 0;
   SinkList sinks;
   scoped_refptr<ConfiguredBuffer> current_buffer;
+  bool disabled = false;
   {
     base::AutoLock auto_lock(lock_);
-    // When the track is disabled, we simply return here.
-    // TODO(xians): Figure out if we should feed zero to sinks instead, in
-    // order to inject VAD data in such case.
-    if (!enabled())
-      return;
-
+    disabled = !enabled();
     capturer = capturer_;
     voe_channels = voe_channels_;
     current_buffer = buffer_;
     sample_rate = current_buffer->params().sample_rate();
     number_of_channels = current_buffer->params().channels();
     number_of_frames = current_buffer->sink_buffer_size();
     sinks = sinks_;
   }
 
   // Push the data to the fifo.
   current_buffer->Push(audio_source);
   // Only turn off the audio processing when the constrain is set to false as
   // well as there is no correct delay value.
   bool need_audio_processing = need_audio_processing_ ?
       need_audio_processing_ : (audio_delay_milliseconds != 0);
-  int current_volume = volume;
+  int current_volume = disabled ? 0 : volume;

[no longer working on chromium, 2013/09/27 20:22:52]

Note that you are fooling the AGC, please test this CL in your local machine and
make sure AGC won't try to set the volume to a value closed to 0 when changing
the track from disable to enable state. 

[ajm, 2013/09/27 22:54:40]

This doesn't look good. You shouldn't be messing around with the volume you
provide, even if disabled.

   while (current_buffer->Consume()) {
     // Feed the data to the sinks.
     for (SinkList::const_iterator it = sinks.begin(); it != sinks.end(); ++it) {
       int new_volume = (*it)->CaptureData(voe_channels,
                                           current_buffer->buffer(),
                                           sample_rate,
                                           number_of_channels,
                                           number_of_frames,
                                           audio_delay_milliseconds,
                                           current_volume,
                                           need_audio_processing,
                                           key_pressed);
-      if (new_volume != 0 && capturer.get()) {
+      if (!disabled && new_volume != 0 && capturer.get()) {
         // Feed the new volume to WebRtc while changing the volume on the
         // browser.
         capturer->SetVolume(new_volume);
         current_volume = new_volume;
       }
     }
   }
 }
 
 void WebRtcLocalAudioTrack::SetCaptureFormat(
@@ skipping 136 matching lines @@
     sinks = sinks_;
     webaudio_source_ = NULL;
     capturer_ = NULL;
   }
 
   for (SinkList::const_iterator it = sinks.begin(); it != sinks.end(); ++it)
     (*it)->Reset();
 }
 
 }  // namespace content