MP3 Files

I load all my MP3's to my SD card for my Nuvi660, click on Travel Kit, MP3 player and make sure the source is pointing to the SD card.

You should be able to load to either internal memory or to SD card. This is how I do it.

I don't think the internal file system of my c550 and your nuvi 660 is very much different.

Internal
If you explore the folders on your 660, you should see a folder named MP3 - you can copy/drag/drop/send to whatever method you prefer into that MP3 folder.
I make folders (and name it accordingly) for each cd that I convert to mp3 and put the corresponding tracks in that folder, then send the entire folder to the garmin->mp3 folder.

SD Card
Do the same with your sd card, create a folder named MP3, then put the mp3's within that. Then move/copy/drag/drop/send to whatever method you like, onto the SD card.

Maybe nickjr can point you to the correct folder path

A couple of things.. I did create folders on my SD card similar to the Nuvi., however for alerts using .mp3's, I copy those voice .mp3's in the same folder that I load my poi's and as stated above I do load them in manual mode and choose certain distances in which I want certain poi files to speak out (ie. I have a poi for Wendys and I created a voice mp3 to go off within 500 feet on any Wendys that I have in my Wendys poi file. and it works flawlessly..

The Nuvi plays only MP3 files. I was having issues when I mistakenly added WMA files. I have since deleted the WMAs and added MP3s, and the Nuvi sounds really good, as long as you're tuned to a station with NO signal. 88.1 FM works fine in my area.