Yeah, I would bet its a DRM problem too. You could try something like Sound Forge to rip them to another format, something like a wave file, and that might remove the DRM. Then, you can rip it back to mp3. There are DRM removal programs as well. There are also file sharing softwares where you can get the files too. This is why I don't like the RIAA. Even when you try to support the artists, you are bootstrapped by file protection, even after you purchase the songs. I had the same problem with a movie that I purchased online.
I don't know if you have a iphone, but that might work, since you can play your purchased library on the iphone and plug it into the AUX port on your car stereo.