It turns out to be easy to make a ringtone from a standard mp3.
I searched and found this YouTube how-to vid. It was quite simple and worked great. The presentation and the accompanying music leave something to be desired, but I took the optimistic view and decided these were an attempt at humor.
Caveat: this technique will only work on standard mp3’s. It will not work on AAC files, and I assume you might experience the same difficulty on tunes downloaded from Napster while they were still using their proprietary format.