If it were the case that Steam went after third-party music, then I'd be in some pretty deep trouble myself.
Like JFD said, it is technically against the TOS. But realistically, you aren't likely to have your mod removed for music or visual assets from a TV show or movie.
The one mod I've seen removed for copyright violations actually contained 3D models ripped directly from another video game, so it's pretty safe to say that going that far is a bad idea, at least.
Edit: Having said this, I have been quite wary about including snippets of popular rock music for my Great Musicians of Rock mod. I planned keep them all shorter than 10 seconds and with rock-bottom audio quality just to be safe (i.e. shorter and of less quality than you can find on the song's Wikipedia page), but still, record companies and their copyrights aren't something I feel like contending with.