Yeah that was possible for a long time... All you do is add a sound element in Unity. You can have it play music and loop it. Pancake's sports world had music and AppleSky's Funsplosive Buildings too. Though there was no way to turn world music off, so Terrium should have an option to turn the world sounds off.
Too bad there weren't more worlds with music or sounds; I only know of those two worlds having music.