I needed the same thing done. I think I may have found an answer for the both of us.
http://www.angelfire.com/journal/compress/
Haven't confirmed if this works. I'll try later. It seems as though you'll need Winamp.
Apparently, there is also a program called Dietmp3 that will compress MP3's as well. It's $19.95 though and I'd rather find a freeware application that does the same thing. Let me know if it works or if you locate another solution.
Thanks.