SpeechSynthesisUtterance: свойство volume

const synth = window.speechSynthesis;

const inputForm = document.querySelector("form");
const inputTxt = document.querySelector("input");
const voiceSelect = document.querySelector("select");

const voices = synth.getVoices();

// ...

inputForm.onsubmit = (event) => {
  event.preventDefault();

  const utterThis = new SpeechSynthesisUtterance(inputTxt.value);
  const selectedOption =
    voiceSelect.selectedOptions[0].getAttribute("data-name");
  for (let i = 0; i < voices.length; i++) {
    if (voices[i].name === selectedOption) {
      utterThis.voice = voices[i];
    }
  }
  utterThis.volume = 0.5;
  synth.speak(utterThis);
  inputTxt.blur();
};