Я пытаюсь объединить N аудио треков в видео файл. Видео в формате MP4, и все аудио m4a.

Вся подготовка работает хорошо, но когда экспорт заканчивается, всегда происходит сбой. Вот мой код:

func mixAudioAndVideo() {
    self.player?.pause()
    let mixComposition = AVMutableComposition()
    let paths = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
    let docsDirect = paths[0]

    for audioTrack in self.audioTracks {
        let musicFile = docsDirect.URLByAppendingPathComponent(audioTrack.audioName)
        let audioAsset = AVURLAsset(URL: musicFile!, options: nil)
        let audioTimeRange = CMTimeRangeMake(audioTrack.audioTime!, audioAsset.duration)
        let compositionAudioTrack:AVMutableCompositionTrack = mixComposition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: kCMPersistentTrackID_Invalid)
        do {
            try compositionAudioTrack.insertTimeRange(audioTimeRange, ofTrack: audioAsset.tracksWithMediaType(AVMediaTypeAudio).first!, atTime: audioTrack.audioTime!)
        } catch let error {
            print(error)
        }
    }

    let videoAsset = AVURLAsset(URL: video!.movieURL, options: nil)
    let videoTimeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
    let compositionVideoTrack = mixComposition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)

    do {
        try compositionVideoTrack.insertTimeRange(videoTimeRange, ofTrack: videoAsset.tracksWithMediaType(AVMediaTypeVideo).first!, atTime: kCMTimeZero)
    } catch let error {
        print(error)
    }

    let videoName = "video\(self.audioTracks.count).mov"
    let outputFilePath = docsDirect.URLByAppendingPathComponent(videoName)

    let assetExport = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality)
    assetExport!.outputFileType = AVFileTypeQuickTimeMovie
    assetExport!.outputURL = outputFilePath!

    assetExport?.exportAsynchronouslyWithCompletionHandler({
        dispatch_async(dispatch_get_main_queue()){
            print("finished exporting \(outputFilePath)")
            print("status \(assetExport?.status)")
            print("error \(assetExport?.error)")
            SVProgressHUD.dismiss()
        }
    })
}

И код, который я получаю:

error Optional(Error Domain=NSURLErrorDomain Code=-1 "unknown error" UserInfo={NSLocalizedDescription=unknown error, NSUnderlyingError=0x170056140 {Error Domain=NSOSStatusErrorDomain Code=-12935 "(null)"}})