Coming to this perhaps too late to be helpful for your jam project, but I think I know how you can avoid the issue in future.
Modern web browsers tend not to allow sound/video to autoplay before the user has interacted with the page. This is intended to prevent companies throwing out annoying ads that basically yell "BUY OUR THING!!!" the moment a page loads, but is a bit of a nuisance for HTML games where the "page" is something the user has specifically opened up to play. The easiest way around it is to make the player click something (perhaps a pre-title/menu "play" button) just to get things going. I had a look at Separaliens on gd.games and I can actually start the menu screen music just by clicking any area that's not a button.
Basically, it seems as though your setup is 100% okay already. The only reason it's not working as expected is the browser's autoplay-blocking behaviour, which is pretty trivial to avoid once you know about it.