Are you using the all_urls permission?
I haven't had issues with the permission given or not but the only real troubleshooting is that as this manipulates elements within webpages (audio/video) and some sites tell Firefox "you're not allowed to".
If all URLs is set on some sites they won't load audio elements at all because of those annoying security policies
On other sites with relaxed security policies the extension will only be able to manipulate audio with all_urls permission because the video is an iframe loaded from a seperate site