Creating our AVMutableComposition and AVPlayer

gistfile1.m

// Setup
   _composition = [AVMutableComposition composition];
   
   _audioMixValues = [[NSMutableDictionary alloc] initWithCapacity:0];
   _audioMixTrackIDs = [[NSMutableDictionary alloc] initWithCapacity:0];
   
   // Insert the audio tracks into our composition
   NSArray* tracks = [NSArray arrayWithObjects:@"track1", @"track2", @"track3", @"track4", nil];
   NSString* audioFileType = @"wav";
   
   for (NSString* trackName in tracks)
   {
      AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:[NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:trackName ofType:audioFileType]]
                                                      options:nil];
      
      AVMutableCompositionTrack* audioTrack = [_composition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];

      NSError* error;
      [audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset.duration)
                          ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0]
                           atTime:kCMTimeZero
                            error:&error];
      
      if (error)
      {
         NSLog(@"%@", [error localizedDescription]);
      }
      
      // Store the track IDs as track name -> track ID
      [_audioMixTrackIDs setValue:[NSNumber numberWithInteger:audioTrack.trackID]
                           forKey:trackName];
      
      // Set the volume to 1.0 (max) for the track
      [self setVolume:1.0f forTrack:trackName];
   }

   // Create a player for our composition of audio tracks. We observe the status so
   // we know when the player is ready to play
   AVPlayerItem* playerItem = [[AVPlayerItem alloc] initWithAsset:[_composition copy]];
   [playerItem addObserver:self
                forKeyPath:@"status"
                   options:0
                   context:NULL];
   
   _player = [[AVPlayer alloc] initWithPlayerItem:playerItem];