Add it to your game engine

extends Node
@onready var music: AudioStreamPlayer = $Music
func _ready():
	music.stream = load("res://music/Purrple Cat - Sands of Time.mp3")
	music.volume_db = -14.0
	music.play()
func _on_music_finished():
	music.play()  # or pick the next track here

using UnityEngine;
public class Soundtrack : MonoBehaviour {
  public AudioClip[] tracks;      // drag your MP3s here
  public AudioSource source;      // volume ~0.3
  int i;
  void Start() { Play(0); }
  void Update() { if (!source.isPlaying) Play((i + 1) % tracks.Length); }
  void Play(int n) { i = n; source.clip = tracks[i]; source.Play(); }
}

// preload
this.load.audio("hub", "assets/music/purrple-cat-hub.mp3");
// create
const music = this.sound.add("hub", { volume: 0.3, loop: true });
this.sound.once("unlocked", () => music.play());  // browsers need a click first

<audio id="bgm" src="music/purrple-cat-hub.mp3" loop></audio>
<script>
  const bgm = document.getElementById("bgm");
  bgm.volume = 0.3;
  document.addEventListener("pointerdown", () => bgm.play(), { once: true });
</script>

function crossfade(from, to, secs = 3) {
  to.volume = 0; to.play();
  const t0 = performance.now();
  (function tick(now) {
    const k = Math.min(1, (now - t0) / (secs * 1000));
    from.volume = (1 - k) * 0.3;
    to.volume = k * 0.3;
    if (k < 1) requestAnimationFrame(tick); else from.pause();
  })(t0);
}