It looks like Chrome has finally implemented a very useful beta feature for all users: It now shows a small speaker icon next to whichever tab is playing sound. While this may not seem like a big deal, it is for tab junkies (ahem) who become quickly frustrated when they can't figure out which tab is blaring an American Idol ad.
The icon looks like this:
This simple feature doesn't go as far as the Chrome extension MuteTab, which lets you select one or multiple tabs to, well, mute. A Firefox extension, Muter, also lets you mute tabs in the browser. However, those extensions don't solve the specific problem of finding a tab that is playing audio. Chrome just fixed that for you.