I was having a similar problem. From what I can tell, SA only easily recognizes .mp3 files. If you have so much as a single .wma file in the user tracks folder, the only thing you'll get are commercials. Clear out everything that isn't an actual .mp3, and then go back in, scan the tracks again, and give it a shot. Worked for me.
Piss-poor planning on your part doesn't constitute an emergency upon mine.
I just got the game to play my m4a files, which is what I encode my songs for my ipod into. I had to download a codec, but once that's installed it's fine - I was reading somewhere that as long as you have codecs installed for all of your files, the game has no problems playing them. I'm working on getting an in-car dvd player for my passengers... stay tuned!
I would go with the combo of the suggestions here. Put mp3s in the usertracks folder, go to audio options to scan the usertracks (I have to do this every time I start the game, do yous?), and I think there's another field you have to click to enable user tracks in the audio options just above the scan option.