Awesome add-on! For me it's perfect, as all I needed was a way to hide a tab that is simply playing a radio feed, so it doesn't get in the way while I work with other tabs.
However, I do understand why other people would want audio from a tab to essentially go mute when quickly hidden.
I'm not sure how difficult it would be to implement a Sound On/Sound Off option, but it would be the ideal way to go to please both users like myself and those who wish to mute any sound.
The only thing I have a problem with is just a minor nit-pick. Unlike other Firefox menu items whose shortcuts I've changed or removed, after I removed the shortcut for Hide Tab, the text in the menus still says "Hide Tab(CTRL+Q)", rather than just "Hide Tab".