SpeechSynthesis.speaking

The speaking read-only property of the SpeechSynthesis interface is a Boolean that returns true if an utterance is currently in the process of being spoken — even if SpeechSynthesis is in a paused state.

var amISpeaking = speechSynthesisInstance.speaking;

A Boolean.

var synth = window.speechSynthesis;

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

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

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

• Web Speech API