This add-on is a proof of concept about the new audio control system of Firefox 30 and it allows users to mix the audio generated by tabs, muting them or changing the volumes.
The audio mixer UI is accessible from the Web Developer menu.
NOTE: If you use Firefox 30, installing this addon, only the visible tab will be allowed to play audio. The others will be muted. This issue is fixed in Firefox 31 in bug 983984 with the support of the default audio channel from preferences.
If you want to know more about the AudioChannel policy, read this page: AudioChannel wiki page.