Avoid clicks when muting/unmuting a voe::Channel.

webrtc/modules/utility/include/audio_frame_operations.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 27 matching lines @@
   static void StereoToMono(const int16_t* src_audio, size_t samples_per_channel,
                            int16_t* dst_audio);
   // |frame.num_channels_| will be updated. This version checks that
   // |num_channels_| is stereo.
   static int StereoToMono(AudioFrame* frame);
 
   // Swap the left and right channels of |frame|. Fails silently if |frame| is
   // not stereo.
   static void SwapStereoChannels(AudioFrame* frame);
 
-  // Zeros out the audio and sets |frame.energy| to zero.
-  static void Mute(AudioFrame& frame);
+  // Conditionally zero out contents of |frame| for implementing audio mute:
+  //  |previous_frame_muted| &&  |current_frame_muted| - Zero out whole frame.
+  //  |previous_frame_muted| && !|current_frame_muted| - Fade-in at frame start.
+  // !|previous_frame_muted| &&  |current_frame_muted| - Fade-out at frame end.
+  // !|previous_frame_muted| && !|current_frame_muted| - Leave frame untouched.
+  static void Mute(AudioFrame* frame, bool previous_frame_muted,
+                   bool current_frame_muted);
 
   static int Scale(float left, float right, AudioFrame& frame);
 
   static int ScaleWithSat(float scale, AudioFrame& frame);
 };
 
 }  // namespace webrtc
 
 #endif  // #ifndef WEBRTC_MODULES_UTILITY_INCLUDE_AUDIO_FRAME_OPERATIONS_H_