My husband put a couple songs in my 660 - using a card that you plug into it. I can't remember which card. According to this site - it says SD card.
https://buy.garmin.com/shop/shop.do?...71&tab=nuvi650
You can play the music directly out of the Garmin itself, but it sounds like crap. You can also use FM modulation I bleieve - which is sketchy. I tried the FM modulation for the bluetooth/ my phone and I just couldn't find a good quality fm station without interferance.
For MP3's - you're better off hooking up directly to your stereo somehow.