It's a weird add-on. It affects the synchronisation between the video and audio across various video streaming sites. It brings in audio delays when active. Moreover, it automatically whitelists a website when its tab is unmuted. So further reloads do not mute the previously unmuted website by default. That setting could however be changed in the "Preferences" section of the add-on. More than relying on third party add-ons.. it's better if websites default to 50% volume while playing any media when they get freshly loaded for the very first time(before cookies save the preferred volume, set manually, for the subsequent loads).

Couldn't unmute all tabs when decided to remove the addon. Spending precious time unmuting by hand. Pretty pissed.