I was under the belief that RM2k3, Advocate's latest translation at least, included MP3 support. Mine certainly does.
There was a utility for importing MP3s into RM2k, the RPG Link Wave Master that was hosted on the old Palace - I have a copy: www.stupid-boy.com/chris/RPGLinkWavMaker.exe This may not be what you're looking for, but it works. If I remember correctly, the first box is the path to your sound directory in your game, the second is the MP3 you wish to import and the 3rd is the new name for the MP3. You'll need to drop the MP3 you want to import into a directory in your game called "MP3".