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.