The main use I have for the new Chrome feature wouldn't be to find out where audio was playing that I hadn't started, it would be to find out which tab was playing audio if I had lots of YouTube tabs open and I couldn't remember which one I had played.

That's the first thing that came to my mind too. I can't remember the last time I had this problem. Click to play + AdBlock = no unknown sounds.

This. Click to play solved this problem since a long time. For me the noise of a website isn't the music, it's the advertising.