Hi Deepak, I'm by no means a JavaScript expert, but hopefully this will help you achieve the desired outcome
Qualtrics.SurveyEngine.addOnload(function()

{

var txtInput = document.querySelector('#Questions');
const removediv = document.querySelector('#Q_lang');
 removediv.remove();
var btnSpeak = document.querySelector('#btnSpeak');
var pausebtn = document.querySelector('#pausebtn');
var resumebtn = document.querySelector('#resumebtn');
var stopbtn = document.querySelector('#stopbtn');
var NextButton = document.querySelector('#NextButton');
var BackButton = document.querySelector('#PreviousButton');
var synth = window.speechSynthesis;
var voices = s];
  var textToSpeak = txtInput.textContent + NextButton.value;

  PopulateVoices();

  if(speechSynthesis !== undefined){
    speechSynthesis.onvoiceschanged = PopulateVoices;
  }

  //start//
  btnSpeak.addEventListener('click', ()=> {    
var toSpeak = new SpeechSynthesisUtterance(textToSpeak);
toSpeak.volume = 1;
toSpeak.pitch = 1;
toSpeak.rate = 1;
  toSpeak.voice = voicesk0]; 
    synth.speak(toSpeak);
  });

  //pause//
  pausebtn.addEventListener('click', ()=> {
    synth.pause();
  });

  //resume//
  resumebtn.addEventListener('click', ()=> {
    synth.resume();
  });

  //stop//
  stopbtn.addEventListener('click', ()=> {
    synth.cancel();
  });

  //stop//
  NextButton.addEventListener('click', ()=> {
    synth.cancel();
  });

  //stop//
  BackButton.addEventListener('click', ()=> {
    synth.cancel();
  });

  function PopulateVoices(){
    voices = synth.getVoices();
    voices.forEach((item, index) => console.log(item.name, index));
  }

});
All I've done is...

1. Add

   removediv.remove();

   immediately below where you've declared your removediv variable


2. Created a new variable

   var textToSpeak = txtInput.textContent + NextButton.value;



3. Referenced this when declaring the toSpeak variable

   var toSpeak = new SpeechSynthesisUtterance(textToSpeak);