what do you think the stock ringtones on the EVO are? they are midi - or they sure as hell sound like it. take a listen.
regardless - i think real song audio mp3 ringtones are pretty cheesy - especially with singing.
The stock ringers are mp3's. Check the directory:
/system/media/audio/ringtones
I have a super mario bro's midi file that's I've been using since whatever I had before my old Treo 650. Sounds awesome on every phone but crappy on the evo. Like they drowned it in reverb. I think I'm going to have to put it on my computer and render it to audio myself. The Evo's just not cutting it. For now - I'm just using the ringtone from the movie Crank.
To contribute to the question:
Since the Evo can use mp3's, you can get them from anywhere. There are several sites online that offer open licensed sound effects (if you don't like music ring tones).
If you want music ringtones, download a wav editor for your PC (ie. Audacity), import the song you want, find a spot in the song you like and cut it. Export it to mp3 and copy it back to your sdcard. Using an instrumental of the song works best IMO.
While midi used to be awesome for ringtones - they sound funny to me on the EVO. I've only tried my Mario midi file as it's the only one I still used...but...