Swift 動画と音声を一つにしたい

Swiftでアプリ開発をしていて、動画と音楽を合成したいと考えています。
下記のように書いて実行すると、エラーがでてしまいます。

swift
1func mergeAudio(audioURL: NSURL, moviePathUrl: NSURL, savePathUrl: NSURL) {
2    var composition = AVMutableComposition()
3    let trackVideo:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID())
4    let trackAudio:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())
5    let option = NSDictionary(object: true, forKey: "AVURLAssetPreferPreciseDurationAndTimingKey")
6    let sourceAsset = AVURLAsset(URL: moviePathUrl, options: option as [NSObject : AnyObject])
7    let audioAsset = AVURLAsset(URL: audioURL, options: option as [NSObject : AnyObject])
8
9    let tracks = sourceAsset.tracksWithMediaType(AVMediaTypeVideo)
10    let audios = audioAsset.tracksWithMediaType(AVMediaTypeAudio)
11
12    if tracks.count > 0 {
13        let assetTrack:AVAssetTrack = tracks[0] as! AVAssetTrack
14        let assetTrackAudio:AVAssetTrack = audios[0] as! AVAssetTrack
15
16        let audioDuration:CMTime = assetTrackAudio.timeRange.duration
17        let audioSeconds:Float64 = CMTimeGetSeconds(assetTrackAudio.timeRange.duration)
18
19        trackVideo.insertTimeRange(CMTimeRangeMake(kCMTimeZero,audioDuration), ofTrack: assetTrack, atTime: kCMTimeZero, error: nil)
20        trackAudio.insertTimeRange(CMTimeRangeMake(kCMTimeZero,audioDuration), ofTrack: assetTrackAudio, atTime: kCMTimeZero, error: nil)
21    }
22
23    var assetExport: AVAssetExportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetPassthrough)
24    assetExport.outputFileType = AVFileTypeMPEG4
25    assetExport.outputURL = savePathUrl
26    self.tmpMovieURL = savePathUrl
27    assetExport.shouldOptimizeForNetworkUse = true
28    assetExport.exportAsynchronouslyWithCompletionHandler { () -> Void in
29        switch assetExport.status {
30        case AVAssetExportSessionStatus.Completed:
31            let assetsLib = ALAssetsLibrary()
32            assetsLib.writeVideoAtPathToSavedPhotosAlbum(savePathUrl, completionBlock: nil)
33            println("success")
34        case  AVAssetExportSessionStatus.Failed:
35            println("failed \(assetExport.error)")
36        case AVAssetExportSessionStatus.Cancelled:
37            println("cancelled \(assetExport.error)")
38        default:
39            println("complete")
40        }
41    }
42
43}

エラーの内容は下記のようなものです。

"failed Error Domain=AVFoundationErrorDomain Code=-11838 "Operation Stopped" UserInfo=0x17da4230 {NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The operation is not supported for this media.}"

原因がわかる方がいましたら教えていただけますでしょうか。
よろしくお願いします。