As @Kokodoko said in the comments of my question, the audio had to be triggered by some form of user interaction. So, I added a button in my HTML and replaced the

document.addEventListener("DOMContentLoaded", () => {})

with

startTimerBtn.addEventListener("click", () => {})

So this is the final code:

const { Howl, Howler } = require("howler");



let i;

let timer;

var alarm = new Howl({

  src: ["alarm.mp3"],

});

let playTimer = true;

const timerDisplay = document.getElementById("timerDisplay");

const startTimerBtn = document.getElementById("startTimer");



startTimerBtn.addEventListener("click", () => {

  i = 5;



  timer = setInterval(() => {

    if (playTimer == true) {

      if (i != 0) {

        timerDisplay.textContent = i;

        i--;

      }



      if (i == 0) {

        alarm.play();

        clearInterval(timer);

      }

    }

  }, 1000);

});

Although this wasn't the solution I intended to get, it suits my needs well.

Also, one small other thing for others using Parcel.js like me. Just in case you don't know, you have to put your audio file in Parcel's dist directory so that it can play. For some reason, it wasn't bundling automatically.