Well, there's another forum I go to that allows html in signatures, however no one actually uses them there. Simply a matter of choice. I think The Sledgehammer used them for a little while back in that time period, as well as other boards we were kinda working on that just died. But I haven't seen them since.
I can link to one though, and show you exactly what it's like. Here's a link.
Click this for example
If you click that, you'll see my current signature, and hover your mouse over it, it plays music. Simple concept of course. And also, it's easy to size the signatures down to proper dimensions, I just didn't bother for this example.
Also notice that if you take your mouse off the signature, the music stops. Simple to control whether or not you're hearing someone's music or not.
I love listening to music through my headphones while I'm on the computer, so sometimes I don't want to hear other people's sigs. Simply put, I don't mouse over them. If I accidentally do for a second, oh well, it was only a split second of sound.
*shrug*