AVAssetExportSessionExport

AVMutableAudioMix *exportAudioMix = [AVMutableAudioMix audioMix];
exportAudioMix.inputParameters = parameters;

AVAssetExportSession *export =
        [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                         presetName:AVAssetExportPresetAppleM4A];
export.outputFileType = @"com.apple.m4a-audio";
export.outputURL      = outputFileUrl;
export.audioMix       = exportAudioMix;

[export exportAsynchronouslyWithCompletionHandler:
    ^(void){ NSLog(@"Tadaaa!"; }       
];