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.
