For those times you know there is a tab making noises that won’t let you peacefully listen a track or a movie (thanks Flash and programmers), comes Muter, a Firefox extension developed by Yuan Xulei (袁徐磊), that adds a button to the Add-on bar that you can click to make Firefox quiet.
Unfortunately it doesn’t let you know what tab is playing, it mutes the whole application, so it is really a shortcut to, on Windows, the Sound Mixer and muting Firefox there. Still, a good help for those who hasn’t opted to make Flash an invite-only guest with Flashblock.
Muter will let you customize the hotkey (Ctrl + Alt + M by default), the icon, and the button placement.
Get Muter from Mozilla Add-ons.