This is the support and feedback thread for Notification Sound.
Why does it not play a sound for every notification?
Sadly the extension can not listen for all notifications websites, extensions or Firefox itself produce. For websites this mainly affects push notifications. Extensions need to explicitly support it. Point the add-on author to https://github.com/freaktechnik/notification-sounds#extension-integration as a guide on how to integrate.
If you think the extension is not playing any sounds you can test it on https://jsfiddle.net/y5gj9tj1/10/. Note that only the top row of buttons should produce a sound (except for the Silent button).
I can’t hear the built in sound but the custom sound on the test page works
Based on user feedback you’re probably using Firefox Pocket Edition and this is a bug on their side. They don’t seem to support FLAC and WAV playback.