                 case .none:
                     if audioFormat == .wav {
                         // repair the WAV header
-                        let wav = NSMutableData(data: data)
+                        var wav = data
                         guard TextToSpeech.isWAVFile(data: wav) else {
                             let failureReason = "Returned audio is in an unexpected format."
                             let userInfo = [NSLocalizedFailureReasonErrorKey: failureReason]

     
     - returns: `true` if the given data is a WAV-formatted audio file; otherwise, false.
     */
    private static func isWAVFile(data: NSData) -> Bool {
        
        // resources for WAV header format:
        // [1] http://unusedino.de/ec64/technical/formats/wav.html

     - returns: An Int initialized by converting the given little-endian byte buffer into
            an unsigned 32-bit integer.
     */
    private static func dataToUInt32(data: Data, offset: Int) -> Int {
        var num: UInt8 = 0
        let length = 4
        let range = Range(uncheckedBounds: (lower: offset, upper: offset + length))
        data.copyBytes(to: &num, from: range)
        return Int(num)
    }
    
