How can merge audio and video in iphone sdk - ios

Question

I am creating a project in which, i merge video and audio using following code in objective c.
AVMutableComposition* mixComposition = [AVMutableComposition composition];
NSString *audio_inputFilePath = [libraryDirectory stringByAppendingPathComponent:#"SelectedSong.wav"];
NSURL *audio_inputFileUrl = [NSURL fileURLWithPath:audio_inputFilePath];
AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:audio_inputFileUrl options:nil];
NSString *videoOutputPath = [libraryDirectory stringByAppendingPathComponent:#"Video.mp4"];
NSURL *video_inputFileUrl = [NSURL fileURLWithPath:videoOutputPath];
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:kCMTimeZero error:nil];
AVMutableCompositionTrack *b_compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[b_compositionAudioTrack insertTimeRange:audio_timeRange ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];
AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
_assetExport.outputFileType = #"com.apple.quicktime-movie";
//_assetExport.outputFileType = AVFileTypeMPEG4 ;
//NSLog(#"support file types= %#", [_assetExport supportedFileTypes]);
_assetExport.outputURL = outputFileUrl;
[_assetExport exportAsynchronouslyWithCompletionHandler:^{
};
but
[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
it return empty array in device.
when check it in simulator it work fine .
so please help me how can resolve it.

Answer 1

In that case
[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
[b_compositionAudioTrack insertTimeRange:audio_timeRange ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:nextClipStartTime error:nil]; this cannot read array.
Because Sound/Audio is not present in the particular file.This Error is comes only on those cases,When sound/Audio is not present in the perticular multimedia file.