Yeah, I figured that MP3 Pro might be an issue.

I was wondering about how to accomplish volume leveling on the fly too. There are some winamp plug-ins that claim to do it and so does Music Match. I know Music Match preprocesses the files and saves information in some way so that it doesn't do it on the fly -- so that kind of implementation would require some server-side code. I don't know about the winamp plug-ins. I will fish around the winamp site and see if I there is any discussion of how it is accomplished.