Eu estou tentando mesclar um arquivo de áudio com um arquivo de vídeo usando:

+ (void)CompileFilesToMakeMovie:(NSString *) audioPath {
NSLog(@"a");
AVMutableComposition* mixComposition = [AVMutableComposition composition];
NSURL *audio_inputFileUrl = [[NSURL alloc] initFileURLWithPath:audioPath];



NSString *videoPath = [[NSBundle bundleForClass:[self class]] pathForResource:@"input" ofType:@"mov"];
NSURL*    video_inputFileUrl = [NSURL fileURLWithPath:videoPath];

NSArray *dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *docsDir = [dirPaths objectAtIndex:0];
NSString *outputFilePath = [docsDir stringByAppendingPathComponent:@"OutputFile.mov"];
NSURL*    outputFileUrl = [NSURL fileURLWithPath:outputFilePath];

if ([[NSFileManager defaultManager] fileExistsAtPath:outputFilePath])
    [[NSFileManager defaultManager] removeItemAtPath:outputFilePath error:nil];
NSLog(@"b");


CMTime nextClipStartTime = kCMTimeZero;

AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:video_inputFileUrl options:nil];
CMTimeRange video_timeRange = CMTimeRangeMake(kCMTimeZero,videoAsset.duration);
AVMutableCompositionTrack *a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[a_compositionVideoTrack insertTimeRange:video_timeRange ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];

 NSLog(@"c");
AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:audio_inputFileUrl options:nil];
CMTimeRange audio_timeRange = CMTimeRangeMake(kCMTimeZero, audioAsset.duration);
AVMutableCompositionTrack *b_compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[b_compositionAudioTrack insertTimeRange:audio_timeRange ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:nextClipStartTime error:nil];
 NSLog(@"d");


AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
_assetExport.outputFileType = @"com.apple.quicktime-movie";
_assetExport.outputURL = outputFileUrl;

[_assetExport exportAsynchronouslyWithCompletionHandler:
 ^(void ) {
     UISaveVideoAtPathToSavedPhotosAlbum(outputFilePath, nil, nil, nil);
 }       
 ];


}

Mas estou recebendo um erro depois que "c" é registrado para 'NSRangeException', razão:

* - [__ NSArrayM objectAtIndex:]: index 0 além dos limites para o array vazio '.

O arquivo de áudio está localizado em

  NSArray *dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
  NSString *docsDir = [dirPaths objectAtIndex:0];
  NSString *soundFilePath = [docsDir stringByAppendingPathComponent:@"%1.iaff"];

Alguma dica? Obrigado.