AVAudioEngine 在扬声器中录制和播放声音

Question

我想用 AVAudioEngine 创建一个 iOS 应用程序，主要功能是从麦克风录制声音并在扬声器上播放声音。我想在扬声器（底部扬声器）和接收器（顶部扬声器）之间切换。我的代码如下，这在接收器上工作正常。我的问题是扬声器无法播放声音。请帮助我，非常感谢！

import  AVFoundation
import UIKit

class XViewController: UIViewController {
    let audioSession = AVAudioSession.sharedInstance()
    var player: AVAudioPlayerNode!
    var engine: AVAudioEngine!

    deinit {
        NotificationCenter.default.removeObserver(self)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()

        setupAudioSession()
        setupNotifications()
        initRecorder()
    }

    func setupAudioSession() {
        do{
            try audioSession.setPreferredSampleRate(16000)
            try self.audioSession.setPreferredOutputNumberOfChannels(1)
            try audioSession.setCategory(.playAndRecord, options: [])
            try audioSession.overrideOutputAudioPort(.none)
            try audioSession.setMode(.voiceChat)
            try audioSession.setActive(true)
        } catch{
            print(error.localizedDescription)
        }
    }

    func initRecorder() {
        player = AVAudioPlayerNode()
        engine = AVAudioEngine()
        let input = engine.inputNode
        engine.attach(player)
        let bus = 0
        let inputFormat = input.inputFormat(forBus: bus)
        engine.connect(player, to: engine.outputNode, format: inputFormat)
        let mixerNode = AVAudioMixerNode()
        engine.attach(mixerNode)
        engine.connect(input, to: mixerNode, format: nil)
        input.installTap(onBus: bus, bufferSize: 2048, format: inputFormat) { (buffer, time) -> Void in
            self.player.scheduleBuffer(buffer)
            print(buffer)
        }
    }

    func start() {
        engine.prepare()
        try! engine.start()
        player.play()
    }

    func playToLoudSpeaker() {
        do{
            try self.audioSession.overrideOutputAudioPort(.speaker)
        } catch{
            print(error.localizedDescription)
        }
    }

    func setupNotifications() {
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(handleRouteChange),
                                               name: AVAudioSession.routeChangeNotification,
                                               object: nil)
    }

    @objc func handleRouteChange(notification: Notification) {
        guard let userInfo = notification.userInfo,
              let reasonValue = userInfo[AVAudioSessionRouteChangeReasonKey] as? UInt,
              let reason = AVAudioSession.RouteChangeReason(rawValue: reasonValue) else {
            return
        }
        // Switch over the route change reason.
        switch reason {
        case .override :
            resume()
        default: break
        }
    }

    func resume() {
        self.engine.stop()
        self.player.stop()
        self.engine.detach(player)

        self.initRecorder()

        engine.prepare()
        try? self.engine.start()
        player.play()
    }

    @IBAction private func startRecorderAndPlay(_ sender: UIButton) {
        start()
    }

    @IBAction private func switchToLoudSpeaker(_ sender: UIButton) {
        playToLoudSpeaker()
    }
}