AVSpeechSynthesizerの音量

SFSpeechRecognizerで音声認識をしながらAVSpeechSynthesizerを使って音声の出力をしようとしたんですけど、音量がすごく小さくなってしまいます。何か解決方法はありませんか？解決方法も色々調べたのですがあまりパッとしなかったので質問させてもらいます。

import UIKit
import Speech

class recording{
    private var recognitionRequest = SFSpeechAudioBufferRecognitionRequest()
    private var speechRecognizer = SFSpeechRecognizer(locale: Locale(identifier: "ja-JP"))!
    private var recognitionTask: SFSpeechRecognitionTask?
    private var audioEngine = AVAudioEngine()
    
    private let synthesizer = AVSpeechSynthesizer()
    public var lang = "ja-JP"
    public var text = "肘です"

        override func viewDidLoad() {
        super.viewDidLoad()
        start()
        }

    func start() {
        recognitionRequest = SFSpeechAudioBufferRecognitionRequest()
        speechRecognizer = SFSpeechRecognizer(locale: Locale(identifier: lang))!
        audioEngine = AVAudioEngine()
        
        let recordingFormat = audioEngine.inputNode.outputFormat(forBus: 0)
        audioEngine.inputNode.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat) { (buffer: AVAudioPCMBuffer, when: AVAudioTime) in
            self.recognitionRequest.append(buffer)
        }
        try! audioEngine.start()
        
        recognitionTask = speechRecognizer.recognitionTask(with: recognitionRequest) { result, error in
            print(result?.bestTranscription.formattedString ?? "")
            let speechresult=result!.bestTranscription.formattedString
            let words = speechresult.suffix(4)
            let trigger = ["ここどこ","これ何","ここは","これなに","これは","これどこ"]
            
            for i in trigger{
                if words.lowercased().contains(i){
                    self.play()
                }
            }
            
            if let error = error {
                print("ERROR!")
                print(error.localizedDescription)
            }
        }
        DispatchQueue.main.asyncAfter(deadline: .now() + 55.0, execute: {
            self.recognitionTask?.cancel()
            self.recognitionTask?.finish()
            self.audioEngine.stop()
            print("RESTART")
            self.start()
        })
    }
        public func play() {
        // 発音を準備する
        //let utterance = AVSpeechUtterance.init(string: self.text)
        let utterance = AVSpeechUtterance.init(string: text)
        let voice = AVSpeechSynthesisVoice.init(language: lang)
        utterance.voice = voice
        // 再生する
        synthesizer.speak(utterance)
    }
}