The speaker thingie is the pager feature. If you have Java enabled on your browser, a little window pops up telling you you've got new comments. It's been around for a while but just now made available to everyone.
P.S. It won't work if you've got a pop-up zapper.
Thank you.
On another thread, I learned that even with my zapper enabled, I can mash CTRL, and the speaker thing will then work.
Hump evening cheers.