Set the audio mix

gistfile1.m

// Build and apply an audio mix using our volume values
- (void)applyAudioMix
{
   AVMutableAudioMix* mix = [AVMutableAudioMix audioMix];
   
   NSMutableArray* inputParameters = [[NSMutableArray alloc] initWithCapacity:0];
   
   [_audioMixTrackIDs enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL*stop) {
      AVAssetTrack* track = [self trackWithId:(CMPersistentTrackID)[(NSNumber*)obj integerValue]];
      
      AVMutableAudioMixInputParameters* params = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:track];
      
      [params setVolume:[[_audioMixValues valueForKey:key] floatValue]
                 atTime:kCMTimeZero];
      
      [inputParameters addObject:params];
   }];
      
   mix.inputParameters = inputParameters;
   
   _player.currentItem.audioMix = mix;
}