So I did a quick web search and found MP3Doctor, it has a normalizer feature. I'm not sure but it looks like the non-pro version is free. Give that one a try first, its at the bottom of the selection HERE
I remember Winamp had a feature called Replay Gain and it allowed you to rip your own CD's and set a uniform gain so that all the CD ripped in the future would be the same volume, but I can't find it right now. But that for future reference when ripping CD's.