I would recommend an Adblocker addon :-)[addons.mozilla.org]

Flash block {web link} Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on it

Based on information in the bug tracking database, when a plugin (Flash, QuickTime, Silverlight) is playing audio, it is difficult for Firefox to detect that and difficult to track down to a particular tab.

For now, the above suggested approaches make the most sense.

To mute ALL of Firefox (if you do not need to hear sound on the current tab), there is a separate extension for that (I haven't tried it myself): https://addons.mozilla.org/firefox/addon/muter/