What window is playing audio? I've got a number of windows open, including several browser windows.  One of them is playing an annoying advertising video.

How can I easily locate which window is playing audio?
Ability to see per-application sound output: Check out https://superuser.com/questions/150304/per-application-volume-settings-possible-on-mac-os-x-snow-leopard

Within Chrome, you can use the Mutetab extension to see what tabs/windows are playing audio.