I hope it isn't inappropriate to post this here: I've built a Chrome extension that could help you find it. http://www.mutetab.com/

Unfortunately, the current version requires that you have the extension prior to opening the tab that plays sound. (But it is fixed in my developer builds and will be available in the next version.)

I had the same problem. I opened up multiple stories from Hacker News and had to click through each of the tabs find out where the music was coming from. Perhaps you could use some sort of javascript mouse event to detect actual user interaction with the opened tab before you start playing the music loop, and yes please make it clearer in your UI on how I can make it stop.

Overall, pretty cool concept/app.