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Google Chrome Team is working on identifying tabs that play audio in order to exclude them from the list of tabs to discard when memory runs out.

In a first step, they've added a small indicator on top of the tab favicon to tell when a tab is actually playing audio.
Youtube video work only if you joined the HTML5 trial at www.youtube.com/html5

You can already see it in the last chromium build (download-chromium.appspot.com).
 
Source: http://codereview.chromium.org/12328027/