title: Switch audio trakcs for AVURLAsset with multiple AVAssetTracks of type audio
body: <p>I have an AVURLAsset with multiple AVAssetTracks of type audio. I would like to be able to allow user to switch between these different audio track by touching a button. It is working to turn volume of 1st track on and off but other tracks are not heard when volume set to 1.0. </p>
  
  <p>Here is code for adjusting the volume of the tracks (sender is a UIButton with tag set to index of asset in audioTracks). </p>
  
  <pre><code>AVURLAsset *asset = (AVURLAsset*)[[player currentItem] asset];
  NSArray *audioTracks = [asset tracksWithMediaType:AVMediaTypeAudio];
  
  
  NSMutableArray *allAudioParams = [NSMutableArray array];
  int i = 0;
  NSLog(@"%@", audioTracks);
  for (AVAssetTrack *track in audioTracks) {
      AVMutableAudioMixInputParameters *audioInputParams =    [AVMutableAudioMixInputParameters audioMixInputParameters];
      float volume = i == sender.tag ? 1.0 : 0.0;
      [audioInputParams setVolume:volume atTime:kCMTimeZero];
      [audioInputParams setTrackID:[track trackID]];
      [allAudioParams addObject:audioInputParams];
      i++;
  }
  AVMutableAudioMix *audioZeroMix = [AVMutableAudioMix audioMix];
  [audioZeroMix setInputParameters:allAudioParams];
  
  [[player currentItem] setAudioMix:audioZeroMix];
  </code></pre>
  
  <p>Do I need to do something to bring the desired track to be the active one?</p>