I don't know about MP3 or WAVs for ringtones, but MIDI files are easy. There are several web sites that offer polyphonic MIDI ringtones out there (including Sprint's PCS Vision site- for $1.50 a ringtone for 90 days!!!). The best deal I've found is
3Gupload.com, you can try out the sounds for free, then if you want some it's a $6 donation for unlimited downloads. They've got thousands of files so you should be able to find some that you like.
Ringtones that play the melody in a very high key seem to work the best. The tiny speaker has no bass so any low sounds are barely audible.