Well, I haven't tried doing it from my Droid, but you can just send it as an e-mail. Send the e-mail to (10 digit phone number)@vzwpix.com and attach the sound. You can send the e-mail from your Droid or your computer or wherever. So if the phone number was 440-555-1234 then you would send it to:
4405551234@vzwpix.com
If I wasn't so lazy I'd go see if I could figure out how to do it directly as an MMS...