The background music is there it's just more faint than the other sound effects. We didn't want it being obnoxious given we didn't have time to add audio settings. It is also a pretty short loop and more ambiance than anything.
I just tested in Chrome and Firefox and I can hear it, were you using a different browser than one of those?
Thanks for double checking. I see that now too. The desktop version is looping fine but web exports do not. Odd stuff. Oh well it wasn't all the interesting of background audio really just there to try to avoid complete silence.
Edit: Looks like it is a reported bug in Godot. That is what I get for not looping the import file. Oh well https://github.com/godotengine/godot/issues/101111
