SpeechSynthesis: свойство speaking

const synth = window.speechSynthesis;

const utterance1 = new SpeechSynthesisUtterance(
  "How about we say this now? This is quite a long sentence to say.",
);
const utterance2 = new SpeechSynthesisUtterance(
  "We should say another sentence too, just to be on the safe side.",
);

synth.speak(utterance1);
synth.speak(utterance2);

const amISpeaking = synth.speaking; // will return true if utterance 1 or utterance 2 are currently being spoken