There's a combination of ways to do this. Its a matter of finding one that fits what you want to do and how much time you want to spend.
I've seen one plugin that adds an HTTP server to Winamp. Using Curl, you could connect to the server and pull the live MP3 info whenever you want. I found this bogs my machine down a little, but if all this computer is doing is dishing MP3s, then its probably not a big deal.
Another option I've used is there was a Winamp plugin which dumped the MP3 info to a file. Then its just a matter of having PHP read the file whenever you want.
Lots and lots of ways to do this. You just need to find a Winamp plugin which you think you can work with...