For Ringers what I do is use a program called Audacity (FREEWARE)
http://audacity.sourceforge.net/
You open your MP3 file in Audacity and then you "crop" your song down to whatever part your want for the ringer. I suggest you don't use any more than 40 seconds worth, since your phone doesn't ring that long anyways.
Then you save the file as an MP3.
Copy to your SD card.
Then go into the Sounds section on the Treo, go to the Manage Tab, then click on Menu on the soft key and select "Add" and then locate the MP3 file on your SD card. The treo will then install the ringer from the SD card over to the Treo's memory.
Works great!