Xcode: How to convert MP4-File to Audio-File

I want to convert a MP4-file from the apps Document folder into an Audio-file (mp3 or m4a). I already tried, but i can not play the converted MP3-file with AVPlayer.

Here is my code:

-(void)convertMP4toMP3withFile:(NSString*)dstPath
{
    NSURL *dstURL = [NSURL fileURLWithPath:dstPath];

AVMutableComposition*   newAudioAsset = [AVMutableComposition composition];

AVMutableCompositionTrack*  dstCompositionTrack;
dstCompositionTrack = [newAudioAsset addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

AVAsset*    srcAsset = [AVURLAsset URLAssetWithURL:dstURL options:nil];
AVAssetTrack*   srcTrack = [[srcAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];


CMTimeRange timeRange = srcTrack.timeRange;

NSError*    error;

if(NO == [dstCompositionTrack insertTimeRange:timeRange ofTrack:srcTrack atTime:kCMTimeZero error:&error]) {
    NSLog(@"track insert failed: %@\n", error);
    return;
}


AVAssetExportSession*   exportSesh = [[AVAssetExportSession alloc] initWithAsset:newAudioAsset presetName:AVAssetExportPresetPassthrough];

exportSesh.outputFileType = AVFileTypeAppleM4A;
exportSesh.outputURL = dstURL;

[[NSFileManager defaultManager] removeItemAtURL:dstURL error:nil];

[exportSesh exportAsynchronouslyWithCompletionHandler:^{
    AVAssetExportSessionStatus  status = exportSesh.status;
    NSLog(@"exportAsynchronouslyWithCompletionHandler: %i\n", status);

    if(AVAssetExportSessionStatusFailed == status) {
        NSLog(@"FAILURE: %@\n", exportSesh.error);
    } else if(AVAssetExportSessionStatusCompleted == status) {
        NSLog(@"SUCCESS!\n");


        NSError *error;
        //append the name of the file in jpg form

        //check if the file exists (completely unnecessary).
        NSString *onlyPath = [dstPath stringByDeletingLastPathComponent];
        NSString *toPathString = [NSString stringWithFormat:@"%@/testfile.m4a", onlyPath];
        [[NSFileManager defaultManager] moveItemAtPath:dstPath toPath:toPathString error:&error];
        [self loadFiles];
    }
}];
}

Has anyone a solution for my problem or can improve my code?