如何将.caf文件转换为.mp3文件？

我使用以下方法在文档文件夹中录制了一些声音

 func record() {
    self.prepareToRecord()
    if let recorder = self.audioRecorder {
        recorder.record()
    }
}

    let recordSettings = [
    AVSampleRateKey : NSNumber(float: Float(44100.0)),
    AVFormatIDKey : NSNumber(int: Int32(kAudioFormatAppleLossless)),
    AVNumberOfChannelsKey : NSNumber(int: 2),
    AVEncoderAudioQualityKey : NSNumber(int: Int32(AVAudioQuality.Medium.rawValue)),
    AVEncoderBitRateKey : NSNumber(int: Int32(320000))
]

    func prepareToRecord() {

    let documentsPath = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as NSString
    let soundFileURL: NSURL? = NSURL.fileURLWithPath("\(documentsPath)/recording.caf")
    print("\(soundFileURL)")

    self.audioRecorder = try! AVAudioRecorder(URL: soundFileURL!, settings: recordSettings)

    if let recorder = self.audioRecorder {
        recorder.prepareToRecord()
    }
}

此方法将音频文件保存为recording.caf到文档目录,但我想将此recording.caf文件转换为mp3以进行进一步操作.

如何将这个.caf文件转换为swift中的.mp3文件？