Issue in merging/mixing of two audio using AVMutableCompositionTrack - ios

Question

it crashes when i try to play the merged audio
I am trying to merging two audio file . They merged successfully, but I am seeing with a problem that when i play that audio with AVAudioPlayer it crashes . also there is a problem with formats as the merge audio only store with .m4a format if i save that audio with .wav format the it crashes.
func merge(audio1: NSURL, audio2: NSURL) {
let finalURL = getMergeFileURL()
let preferredTimeScale : Int32 = 100
//This object will be edited to include both audio files
let composition = AVMutableComposition()
let compositionAudioTrack1:AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())
//let url1 = audio1
let avAsset1 = AVURLAsset(url: audio1 as URL, options: nil)
let tracks1 = avAsset1.tracks(withMediaType: AVMediaTypeAudio)
let assetTrack1:AVAssetTrack = tracks1[0]
let duration1: CMTime = CMTimeMakeWithSeconds(30.0, preferredTimeScale)
let startCMTime = CMTimeMakeWithSeconds(Double(30.0), preferredTimeScale)
let timeRange1 = CMTimeRangeMake(startCMTime, duration1)
let compositionAudioTrack2:AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID())
//let url2 = audio2
let avAsset2 = AVURLAsset(url: audio2 as URL, options: nil)
let tracks2 = avAsset2.tracks(withMediaType: AVMediaTypeAudio)
let assetTrack2:AVAssetTrack = tracks2[0]
let duration2: CMTime = CMTimeMakeWithSeconds(30.0, preferredTimeScale)
let startCMTime2 = CMTimeMakeWithSeconds(Double(30.0), preferredTimeScale)
let timeRange2 = CMTimeRangeMake(startCMTime, duration1)
//Insert the tracks into the composition
do {
try compositionAudioTrack1.insertTimeRange(timeRange1, of: assetTrack1, at: kCMTimeZero)
try compositionAudioTrack2.insertTimeRange(timeRange2, of: assetTrack2, at: duration1)
} catch {
print(error)
}
//Perform the merge
let assetExport = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A)
assetExport!.outputFileType = AVFileTypeAppleM4A
assetExport!.outputURL = finalURL as URL // final url is the url of that merged file
assetExport!.exportAsynchronously(completionHandler: {
switch assetExport!.status{
case AVAssetExportSessionStatus.failed:
print("failed \(assetExport!.error)")
case AVAssetExportSessionStatus.cancelled:
print("cancelled \(assetExport!.error)")
default:
print("complete")
}
})
}