AVAsset 'tracksWithMediaType‘返回空数组？(音频)

Question

我想用AVAssetExportSession.When合成两个音频我用AVAsset用'tracksWithMediaType:AVMediaTypeAudio‘得到音轨，我得到一个空数组。这些音频文件是由'AVAudioRecorder‘录制的，并且它们有.aac扩展名。我用来获取跟踪的代码：

 AVAsset * asset = [AVAsset assetWithURL:[NSURL URLWithString:model.localPath]];

 NSArray * tracks = [asset tracksWithMediaType:AVMediaTypeAudio];

 AVAssetTrack * track = nil;
 if (tracks.count) {
     track = tracks[0];
 }

初始化AVAudioRecorder的设置

AVAudioSession * audioSession = [AVAudioSession sharedInstance];
[audioSession setCategory:AVAudioSessionCategoryRecord error:nil];
[audioSession setActive:YES error:nil];


NSMutableDictionary *recordSettings = [[NSMutableDictionary alloc] init];
[recordSettings setValue :[NSNumber numberWithInt:kAudioFormatMPEG4AAC] forKey: AVFormatIDKey];
[recordSettings setValue :[NSNumber numberWithFloat:11025.0] forKey: AVSampleRateKey];//44100.0
[recordSettings setValue :[NSNumber numberWithInt:2] forKey: AVNumberOfChannelsKey];
[recordSettings setValue:[NSNumber numberWithInt:AVAudioQualityMax] forKey:AVEncoderAudioQualityKey];

我还尝试使用其他.mp3文件来获取跟踪，但它是一个空数组。音频文件的本地路径为：

  /var/mobile/Containers/Data/Application/DDAB48C4-A233-4B4C-B3B7-5F66AE076239/Documents/media/1497494042.aac