In horror games, music can be especially problematic: you don't want the music to be more scary than your game, you don't want to play something too light that spoils the mood, and you don't want to rely on "Tubular Bells" again.
The solution that avoids all those pitfalls is to DIY your own horrorific ambient soundtrack. It sounds like a lot to ask, doesn't it? "I have to make my own adventures, draw my own maps, learn the rules of the game, and now I have to make music too?"
You're going to be surprised by how easy this is.
1) Download Audacity, a free audio editing program here. Install the program.
2) Start Audacity. File-->Open--> and load any music mp3 you've already got on your computer. Seriously, it doesn't matter if your source file is Carly Rae Jepsen or Rammstein, this process will make it into a long, creepy ambient track.
3) Effect-->Change Speed--> and reduce speed by about 90% or so. It will take a while to apply the change, as this makes the track significantly longer.
4) File-->Export the file as a mp3 at the bitrate of your choice. You're done.
The end result will be a slow, eerie track full of ominous rumblings, unexpected noisome growls, and Cthulhoid mutterings. If there was a pronounced beat in the original file, it will now likely be at a dragging, funereal tempo that sounds like footsteps through an abandoned slaughterhouse.
Play your new soundtrack at a low volume in the background for your next Call of Cthulhu or Don't Rest Your Head game and marvel at your ingenuity.
As an example of what the end product might sound like, here's one I made from a Combichrist song.
As another example, here's another one made from a Smiths song.