And you are right Keleth, for Horror I try very hard to use music to control the mood. I actually tend to use video game music a lot. Amnesia: Dark Descent is a great sound track, for example, that I often use. I have a few other miscellaneous soundtracks on iTunes that I don't know what the game is they are from. My back up is to go to youtube and search "horror game music" and similar things then listen for the mood I'm trying to capture.
One fun thing to do with audio in a horror game you can do if you have a recurring horror. Designate a soundtrack (say this one:
Amneisa - Attack Guardian), and on the first few encounters (which, by good horror telling tropes, you don't fully reveal or let them properly identify) play that music. Then, sometime later, during a regular scene, have the horror's music on the soundtrack list and once it starts playing, if you have done it right, the players will suddenly become vigilant. Verify their suspicions by making it another horror encounter (this can make things fun for the GM because you don't necessarily know what the players will be doing when the horror music starts - if you're the kind of GM that likes improv).
