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.

You can select a different sound file to play in the extension options. If you don't know how to find that, please feel free to reach out to me at the provided support site.

Hi
This is likely due to the codec of the sound file not being supported when playing from an extension unfortunately. Apparently installed something like VLC can help, since it adds codecs to the system.