Swift Слияние аудио и видео Swift3

У меня есть одно видео, и я объединяю другой звук с этим видео, поэтому прямо сейчас у меня есть два аудио с этим видео, один - оригинальный звук, а другой - собственный звук видео. Теперь проблема в том, что я могу установить громкость для собственного звука видео, но не могу установить громкость внешнего звука. Я использовал следующий код, чтобы сделать то же самое.

func mergeAV(videoUrl: URL, audioUrl: URL, shouldFlipHorizontally: Bool = false) {
    print("\(#function)")
    process_merge.startAnimating()

    let mixComposition = AVMutableComposition()
    var mutableCompositionVideoTrack = [AVMutableCompositionTrack]()
    var mutableCompositionAudioTrack = [AVMutableCompositionTrack]()
    var mutableCompositionAudioOfVideoTrack = [AVMutableCompositionTrack]()

    let aVideoAsset = AVAsset(url: videoUrl)
    let aAudioAsset = AVAsset(url: audioUrl)

    let compositionAddVideo = mixComposition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: kCMPersistentTrackID_Invalid)
    let compositionAddAudio = mixComposition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: kCMPersistentTrackID_Invalid)
    let compositionAddAudioOfVideo = mixComposition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: kCMPersistentTrackID_Invalid)

    let aVideoAssetTrack: AVAssetTrack = aVideoAsset.tracks(withMediaType: AVMediaType.video)[0]
    let aAudioOfVideoAssetTrack: AVAssetTrack? = aVideoAsset.tracks(withMediaType: AVMediaType.audio).first
    let aAudioAssetTrack: AVAssetTrack = aAudioAsset.tracks(withMediaType: AVMediaType.audio)[0]

    compositionAddVideo?.preferredTransform = aVideoAssetTrack.preferredTransform

    mutableCompositionVideoTrack.append(compositionAddVideo!)
    mutableCompositionAudioTrack.append(compositionAddAudio!)
    mutableCompositionAudioOfVideoTrack.append(compositionAddAudioOfVideo!)

    do {
        try mutableCompositionVideoTrack[0].insertTimeRange(CMTimeRangeMake(start: CMTime.zero, duration: aVideoAssetTrack.timeRange.duration), of: aVideoAssetTrack, at: CMTime.zero)

        try mutableCompositionAudioTrack[0].insertTimeRange(CMTimeRangeMake(start: CMTime.zero, duration: aVideoAssetTrack.timeRange.duration), of: aAudioAssetTrack, at: CMTime.zero)

        if let aAudioOfVideoAssetTrack = aAudioOfVideoAssetTrack {
            try mutableCompositionAudioOfVideoTrack[0].insertTimeRange(CMTimeRangeMake(start: CMTime.zero, duration: aVideoAssetTrack.timeRange.duration), of: aAudioOfVideoAssetTrack, at: CMTime.zero)
        }

    } catch {
        print(error.localizedDescription)
    }

    // Exporting
    let fileManager = FileManager.default
    let documentsDirectory = fileManager.urls(for: .documentDirectory, in: .userDomainMask)[0]
    let savePathUrl = documentsDirectory.appendingPathComponent("MergeAudioVideo.mp4")

    do {
        // delete old video
        try FileManager.default.removeItem(at: savePathUrl)
    } catch {
        print(error.localizedDescription)
    }

    var videoVolume : Float = 0.0
    var audioVolume : Float = 0.0
    videoVolume = self.vol_Video.value
    audioVolume = self.vol_Audio.value

    let audioMix: AVMutableAudioMix = AVMutableAudioMix()
    var audioMixParam: [AVMutableAudioMixInputParameters] = []

    let videoParam: AVMutableAudioMixInputParameters = AVMutableAudioMixInputParameters(track: aVideoAssetTrack)
    videoParam.trackID = (compositionAddVideo?.trackID)!

    let musicParam: AVMutableAudioMixInputParameters = AVMutableAudioMixInputParameters(track: aAudioAssetTrack)
    musicParam.trackID = (compositionAddAudio?.trackID)!

    videoParam.setVolume(videoVolume, at: CMTime.zero)
    musicParam.setVolume(audioVolume, at: CMTime.zero)

    videoParam.setVolumeRamp(fromStartVolume: videoVolume, toEndVolume: videoVolume, timeRange: aVideoAssetTrack.timeRange)
    musicParam.setVolumeRamp(fromStartVolume: videoVolume, toEndVolume: audioVolume, timeRange: aAudioAssetTrack.timeRange)

    audioMixParam.append(musicParam)
    audioMixParam.append(videoParam)

    audioMix.inputParameters = [musicParam,videoParam]

    let assetExport: AVAssetExportSession = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality)!
    assetExport.outputFileType = AVFileType.mp4
    assetExport.outputURL = savePathUrl
    assetExport.shouldOptimizeForNetworkUse = true
    assetExport.audioMix = audioMix

    print("Export Action")
    assetExport.exportAsynchronously { () -> Void in
        self.process_merge.stopAnimating()

        switch assetExport.status {
        case AVAssetExportSession.Status.completed:
            print("Merge video+Audio URL : \(savePathUrl)")
            print("Export : success")
            self.showMess(strMess: "Success...\nMerge video+Audio URL: \(savePathUrl)", isAlert: false)
            break

        default:
            print("Error : unknown")
            self.showMess(strMess: "Something was wrong. Please try again.\nError: \(assetExport.error?.localizedDescription ?? "unknown")", isAlert: true)
            break
        }
    }
}

Как я могу это решить? Пожалуйста помоги!