AVMutableComposition with AKAudioFiles just exports the first track - swift

Question

I recently have struggling to get AVMutableComposition working in my synthesizer app. I implemented a function like the one below without Audiokit and it worked perfectly fine. However, when I run the function below I only can here the first file in the files arrray in the outputed AKAudioFile. Is this a problem with how AKAudioFiles are copied or how AVMutableComposition works?
private func createCombinedRecordingFrom(_ files: [AKAudioFile], completion: #escaping (AKAudioFile?) -> ()) {
let composition = AVMutableComposition()
for file in files {
let asset = file.avAsset
let timeRange = CMTimeRange(start: kCMTimeZero, duration: asset.duration)
let track = composition.addMutableTrack(withMediaType: .audio,
preferredTrackID: kCMPersistentTrackID_Invalid)
do {
try track?.insertTimeRange(timeRange, of: asset.tracks[0], at: kCMTimeZero)
} catch {
print(error.localizedDescription)
completion(nil)
}
}
let assetExport = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A)!
assetExport.audioTimePitchAlgorithm = .spectral
assetExport.outputFileType = .m4a
let temporaryURL = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(UUID().uuidString).appendingPathExtension("m4a")
assetExport.outputURL = temporaryURL
assetExport.exportAsynchronously() {
do {
let audioFile = try AKAudioFile(forReading: temporaryURL)
completion(audioFile)
} catch {
print(error.localizedDescription)
completion(nil)
}
}
}