swift 将录制的多个aac音频合并为m4a格式音频

func mergeAudio() {
        let composition = AVMutableComposition()
        
        for i in 0 ..< audioLocalUrls.count {
            
            let compositionAudioTrack :AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())
            
            let asset = AVURLAsset(url: URL.init(fileURLWithPath: audioLocalUrls[i]))
            
            let track = asset.tracks(withMediaType: AVMediaTypeAudio)[0]
            
            var timeRange:CMTimeRange
            timeRange = CMTimeRange(start: CMTimeMake(0, 600), duration: track.timeRange.duration)
            
            try! compositionAudioTrack.insertTimeRange(timeRange, of: track, at: composition.duration)
        }
        
        let documentDirectoryURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first! as NSURL
        let stringDate = BaseController().getCurrentTime()
        self.mergeAudioURL = (documentDirectoryURL.appendingPathComponent(BaseController().randomMD5(str: stringDate) + ".m4a")! as URL as NSURL) as URL!
        
        let assetExport = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A)
        assetExport?.outputFileType = AVFileTypeAppleM4A
        assetExport?.outputURL = mergeAudioURL as URL
        
        assetExport?.exportAsynchronously(completionHandler:
            {
                switch assetExport!.status
                {
                    case AVAssetExportSessionStatus.failed:
                        print("failed \(assetExport?.error)")
                    case AVAssetExportSessionStatus.cancelled:
                        print("cancelled \(assetExport?.error)")
                    case AVAssetExportSessionStatus.unknown:
                        print("unknown\(assetExport?.error)")
                    case AVAssetExportSessionStatus.waiting:
                        print("waiting\(assetExport?.error)")
                    case AVAssetExportSessionStatus.exporting:
                        print("exporting\(assetExport?.error)")
                    default:
                        print("success\(assetExport?.error)")
                        
                    
                    for i in 0..<self.audioLocalUrls.count{
                        let fileManager = FileManager.default
                        try! fileManager.removeItem(at: URL.init(fileURLWithPath: self.audioLocalUrls[i]))
                    }
                }
        })
    }