Hello davetik,

This is Kevin from palmOne. Your Treo can use midis as ringtones. There may be third-party solutions for other file formats, but I'm not very familiar with them. You can either email the midis to yourself or download them directly. There is a good step-by-step guide to all this in the palmOne Knowledge Library article "Get MIDI ringtones on your Treo smartphone" here.
Hope that helps.

Sincerely
Kevin