During cascading 2 audio files tracksWithMediaType always returns empty array

Question

I have 2 audio files audioFile1.m4a and audioFile2.m4a. I want to create a cascaded audio combined.m4a which contains audioFile1.m4a and audioFile2.m4a, one after another. I used below code and my problem is tracksWithMediaType always returns empty array. audioIds contains audio file names.

    NSError *error = nil;
    BOOL ok = NO;


    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,    NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];


    CMTime nextClipStartTime = kCMTimeZero;
    //Create AVMutableComposition Object.This object will hold our multiple AVMutableCompositionTrack.
    AVMutableComposition *composition = [[AVMutableComposition alloc] init];

    AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

    for (int i = 0; i< [audioIds count]; i++) {
        NSString *audioFileName = [audioIds objectAtIndex:i];

        //Build the filename with path

        NSString *soundOne = [documentsDirectory stringByAppendingPathComponent:audioFileName];

        NSLog(@"voice file - %@",soundOne);

        NSURL *url = [NSURL fileURLWithPath:soundOne];
        AVAsset *avAsset = [AVURLAsset URLAssetWithURL:url options:nil];
        NSArray *tracks = [avAsset tracksWithMediaType:AVMediaTypeAudio];
        if ([tracks count] == 0)
            return;
        CMTimeRange timeRangeInAsset = CMTimeRangeMake(kCMTimeZero, [avAsset duration]);
        AVAssetTrack *clipAudioTrack = [[avAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
        ok = [compositionAudioTrack insertTimeRange:timeRangeInAsset  ofTrack:clipAudioTrack atTime:nextClipStartTime error:&error];
        if (!ok) {
            NSLog(@"Current Video Track Error: %@",error);
        }
        nextClipStartTime = CMTimeAdd(nextClipStartTime, timeRangeInAsset.duration);
    }

    // create the export session
    // no need for a retain here, the session will be retained by the
    // completion handler since it is referenced there
    AVAssetExportSession *exportSession = [AVAssetExportSession
                                           exportSessionWithAsset:composition
                                           presetName:AVAssetExportPresetAppleM4A];
    if (nil == exportSession) return ;

    __block NSString *soundOneNew = [documentsDirectory stringByAppendingPathComponent:@"combined.m4a"];
    //NSLog(@"Output file path - %@",soundOneNew);

    // configure export session  output with all our parameters
    exportSession.outputURL = [NSURL fileURLWithPath:soundOneNew]; // output path
    exportSession.outputFileType = AVFileTypeAppleM4A; // output file type

    // perform the export
    [exportSession exportAsynchronouslyWithCompletionHandler:^{

        if (AVAssetExportSessionStatusCompleted == exportSession.status)
        {
            NSLog(@"AVAssetExportSessionStatusCompleted");
        } else if (AVAssetExportSessionStatusFailed == exportSession.status) {
            // a failure may happen because of an event out of your control
            // for example, an interruption like a phone call comming in
            // make sure and handle this case appropriately
            NSLog(@"AVAssetExportSessionStatusFailed");
        } else {
            NSLog(@"Export Session Status: %ld", (long)exportSession.status);
        }
    }];