How to convert MP3 from TTS ( Text to Speech )

Question

I have made text to speech functionality using the link mentioned below:-

MDN - https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynthesisUtterance

MDN Example - https://mdn.github.io/web-speech-api/speak-easy-synthesis/

I want to convert it into an MP3 file with a specific voice i.e mentioned in the Select Voice dropdown. How can I achieve that?

Note:- I want to achieve this functionality using JS or PHP

Below is my code

var inputSpeakBtn = document.querySelector('.btn-test-voice');
        var inputTxt = document.querySelector('#your-text');
        var voiceSelect = document.querySelector('#new-voice-list');
    
        var synth = window.speechSynthesis;
        
        setTimeout(() => {
            
            var voices = synth.getVoices();
            
            for (let index = 0; index < voices.length; index++) {
                const voicesElement = voices[index];
    
                var option = document.createElement('option');
                option.textContent = voicesElement.name + ' (' + voicesElement.lang + ')';
                option.value = index;

                voiceSelect.appendChild(option);
            }
            
            inputSpeakBtn.addEventListener('click', function() {
                var utterThis = new SpeechSynthesisUtterance(inputTxt.value);
                utterThis.voice = voices[voiceSelect.value];
                synth.speak(utterThis);
                inputTxt.blur();
            })
        }, 1000);

<div class="form-group col-md-12 form-new-voice use-tts">

    <label for="">Your Text</label>
    <input id="your-text" type="text" class="form-control" placeholder="">

    <label for="" class="mt-2">Select Voice</label>
    <select id="new-voice-list" class="form-control">
    </select>
                                                                
   <button class="btn btn-primary btn-test-voice mt-2">Test Voice</button>
   <button class="btn btn-primary btn-download mt-2">Download</button>
</div>