Merging two wav files in Swift

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        var audio1 = NSURL(fileURLWithPath:  NSBundle.mainBundle().pathForResource("Buildup1", ofType: "wav")!)!
        var audio2 = NSURL(fileURLWithPath:  NSBundle.mainBundle().pathForResource("Verse1", ofType: "wav")!)!
        var audio3 = NSURL(fileURLWithPath:  NSBundle.mainBundle().pathForResource("Drop1", ofType: "wav")!)!
        self.concatenate(audio1, audio2Url: audio2, audio3Url: audio3)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    func concatenate(audio1Url: NSURL, audio2Url:  NSURL, audio3Url: NSURL) {


        var error:NSError?

        var ok1 = false
        var ok2 = false
        var ok3 = false


        //var documentsDirectory:String = paths[0] as! String

        //Create AVMutableComposition Object.This object will hold our multiple AVMutableCompositionTrack.
        var composition = AVMutableComposition()
        var compositionAudioTrack:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())
        //var compositionAudioTrack2:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())

        //create new file to receive data
        var documentDirectoryURL = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first! as! NSURL
        var fileDestinationUrl = documentDirectoryURL.URLByAppendingPathComponent("resultmerge.wav")
        println(fileDestinationUrl)


        var avAsset1 = AVURLAsset(URL: audio1Url, options: nil)
        var avAsset2 = AVURLAsset(URL: audio2Url, options: nil)
        var avAsset3 = AVURLAsset(URL: audio3Url, options: nil)

        var tracks1 =  avAsset1.tracksWithMediaType(AVMediaTypeAudio)
        var tracks2 =  avAsset2.tracksWithMediaType(AVMediaTypeAudio)
        var tracks3 =  avAsset3.tracksWithMediaType(AVMediaTypeAudio)

        var assetTrack1:AVAssetTrack = tracks1[0] as! AVAssetTrack
        var assetTrack2:AVAssetTrack = tracks2[0] as! AVAssetTrack
        var assetTrack3:AVAssetTrack = tracks3[0] as! AVAssetTrack


        var duration1: CMTime = assetTrack1.timeRange.duration
        var duration2: CMTime = assetTrack2.timeRange.duration
        var duration3: CMTime = assetTrack3.timeRange.duration



        var timeRange1 = CMTimeRangeMake(kCMTimeZero, duration1)
        var timeRange2 = CMTimeRangeMake(kCMTimeZero, duration2)
        var timeRange3 = CMTimeRangeMake(kCMTimeZero, duration3)


        ok1 = compositionAudioTrack.insertTimeRange(timeRange1, ofTrack: assetTrack1, atTime: kCMTimeZero, error: nil)
        if ok1 {

            ok2 = compositionAudioTrack.insertTimeRange(timeRange2, ofTrack: assetTrack2, atTime: duration1, error: nil)

            if ok2 {
                ok3 = compositionAudioTrack.insertTimeRange(timeRange3, ofTrack: assetTrack3, atTime: CMTimeAdd(duration1, duration2), error: nil)

            }
        }

        //AVAssetExportPresetPassthrough => concatenation
        var assetExport = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A)
        assetExport.outputFileType = AVFileTypeAppleM4A
        assetExport.outputURL = fileDestinationUrl
        assetExport.exportAsynchronouslyWithCompletionHandler({
            switch assetExport.status{
            case  AVAssetExportSessionStatus.Failed:
                println("failed \(assetExport.error)")
            case AVAssetExportSessionStatus.Cancelled:
                println("cancelled \(assetExport.error)")
            default:
                println("complete")
                var audioPlayer = AVAudioPlayer()
                audioPlayer = AVAudioPlayer(contentsOfURL: fileDestinationUrl, error: nil)
                audioPlayer.prepareToPlay()
                audioPlayer.play()
            }

        })

    }
}