Dumb question, I’m sure. But my Mac won’t even recognize one CD from a two-disc set (the other plays fine, all the tracks come up, i can copy it onto iTunes, etc.), but it will play in my stereo. WTF? Is this some sort of copy protection thing? (It’s from 2003, if that helps)
Is it clean and scratch free? Has it ever played before? 
Sometimes a CD player will play a scratched disc, but the computer won’t or vice versa.
What CD is it?
I had a similiar problem with certain 2 CD albums.
I’m not sure this will help you. Sony is in deep ____ for installing an anti piracy software as a rootkit. and it can cause problems on computers, but not on cd players. This link points to the latest on that subject. The site has at least four articles, and one of them has links to some pages discussing this in depth. Any person with these cd’s can exchange them for non copyrighted versions that do not alter your operating system’s kernel code. This alteration can be exploted to hide other peoples malicious software also.
http://www.aunty-spam.com/sony-latest-statement-on-rootkit-recalls-cds-offers-exchange/
I doubt it’s DRM, that would likely prevent both discs from playing , not just one. Could conceivably be faulty DRM, I suppose. Still, sounds more likely to me that the one disc is simply scratched up too much or damaged in some way.
List of malware-infected CDs put out by Sony. Is it on this list?
It can’t be scratches- the bottom looks pristine. It’s a next-to-new promo copy of a Coyote Shivers album that I got off of Amazon. And it’s an indie release- Foodchain Records- so the Sony thing can’t be it.