I think this is impossible to do if the audio is being played via java/flash/etc as the plugin bypasses the browser completely. Here's a comment from the google chrome team from a while back: http://www.reddit.com/r/IAmA/comments/gdyun/iama_we_are_thre...

It should however be possible for webpages that use browser APIs to play audio (e.g. via HTML5).

I've often thought that it would be good if the (otherwise excellent) Chrome task manager included an indication of which tabs were currently generating sound.

It's very annoying when an advert or a forgotten youtube embed suddenly starts making noises on a tab I'm not looking at and I end up hunting through several tabs and closing innocent unread tabs trying to stop the noise.

(I noticed the ▶ in the title bar a while ago and like it a lot. Mixcloud.com also does a nice thing where the favicon shows you how far through a mix you are and whether it's playing or paused.)