AVAssetExportSession - AVMutableCompositionTrack - exported output file is empty

I try to add a .mp3-file to an AVMutableCompositionTrack and after that I want to export the new file. The Problem is: The generated file exists after exporting but it is empty and can not be played. Does someone see the error in my code?

AVMutableComposition *saveComposition = [AVMutableComposition composition];
 NSArray *docpaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
 NSString *tempPath = [docpaths objectAtIndex:0];

 NSLog(@"Temporary Path: %@", tempPath);

 NSString *audioPath = [[NSBundle mainBundle] pathForResource: @"1" ofType: @"mp3"];
 NSURL *audioUrl = [[NSURL alloc] initFileURLWithPath:audioPath];
 AVURLAsset *audio = [AVURLAsset URLAssetWithURL:audioUrl options:nil];
 NSLog(@"%@", audio);
 [audioUrl release];
 AVMutableCompositionTrack *compositionAudioTrack = [saveComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
 AVAssetTrack *clipAudioTrack = [[audio tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

 [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [audio duration])  ofTrack:clipAudioTrack atTime:kCMTimeZero error:nil];

 NSString *path = [tempPath stringByAppendingPathComponent:@"mergedaudio.m4a"];
 if([[NSFileManager defaultManager] fileExistsAtPath:path])
 {
  [[NSFileManager defaultManager] removeItemAtPath:path error:nil];
 }
 NSURL *url = [[NSURL alloc] initFileURLWithPath: path];

 AVAssetExportSession *exporter = [[[AVAssetExportSession alloc] initWithAsset:saveComposition presetName:AVAssetExportPresetAppleM4A] autorelease];
 exporter.outputURL=url;
 [exporter setOutputFileType:@"com.apple.m4a-audio"];

 NSLog(@"%@", [exporter supportedFileTypes]);
 exporter.outputFileType=[[exporte开发者_如何学编程r supportedFileTypes] objectAtIndex:0];

 [exporter exportAsynchronouslyWithCompletionHandler:^{

 }];

Thank you in advance !

Like I wrote in the comments it had something to do with the different file formats. I changed my files to .m4a and also the code - so everything (the sources and the targets of this operation) is .m4a related and it works.

by the way: I tried also working with .wav-files, but there are strange things happening while the operations with wav. I don't recommend it.