Notifications generated by service workers don't work. This is for sure what youtube uses, since you also get notifications from youtube when the tab is not open. Sadly there is no way for the extension to intercept these notifications.

Thanks for this - what is essentially a bug report - I was not aware that I had issues with not having the proper playback permissions. I thought that extensions were made exempt when they were initially introduced. Since the extension can't ask for permission like a webpage can. However, I'm sadly unable to reproduce the exact issue with the latest Firefox version (78.0.1). For me the default sound doesn't work, but selecting a custom sound works. Sadly playing sounds from an extension is a bit cumbersome, and as many reviews show, a bit error prone - however the errors depend a lot on OS, Firefox version, and other system variables that I haven't fully narrowed down.

the Facebook and Twitter notifications likely come from Service Workers, which also work when the tab is not open. As described in the description, it is sadly impossible to play a sound for those (in a generic way).