But anyway, major browsers (other than IE) -- even when tab per process -- only use a single Flash process for all tabs, so you cannot distinguish at the operating system level. Chrome is now able to show audio indicators because Flash is embedded via a new kind of plug-in (Pepper) which allows them to track it.
Since Firefox knows which tabs embed Flash content and whether the Flash plugin process is playing audio, Firefox might be able to guess which tab is playing Flash audio. For example, if there is only one tab with Flash content. :)
But due to ads, if many tabs are open then many will have Flash on them. Run Chrome, install my MuteTab extension (http://www.mutetab.com/), open a few tabs, and look at the extension popup to get an idea of how pervasive Flash is. (However, if you also run Adblock then it becomes a little more reasonable, since you've greatly reduced the number of tabs playing with Flash on them.)
Yeah, you could look at which tabs were recently opened when sound started playing. But then once sound is playing, you won't be able to detect if another tab that plays sound was opened. And the sound might have come from a tab that had been open for awhile.
In Win7 you get audio volume per process option when you click the speaker tray icon. So the OS does know the process source of audio
Since Chrome is multi-process I think it's easy to identify the noisy process.
Just an idea.