The incredible soundtrack for the game was composed by Kyle Gabler, largely using computer instruments, and provides an atmospheric and exciting backdrop to play.
Following overwhelming fan requests, Kyle released a freely downloadable soundtrack in January 2009, consisting of all the songs from the game, many in expanded form.
Although much of the music was composed for the game, several of the songs were originally written for his earlier projects, but expanded and improved for the game. Notes on the songs can be found on his webpage. You can also find some of the songs on his old portfolio.
Sebastian Wolff has made piano sheet music of most of the game's music. You can find it, in various formats along with MP3s and MIDI files, on his site.
Check out this YouTube video of him playing through the complete medley: