This extension is a new soundtrack for the game. It's made up completely of music by Marc A. Pullen, one of my favourite artists, who incidentally allows his music to be used for free. It's a genre of music called "math metal", which has some unusual rhythms and structure that makes it suitable for stopping and starting often like the official soundtrack does.
Note: The official soundtrack doesn't seem to agree with this, so make sure that it isn't in your collection folder and that it isn't downloaded again (meaning you may have to play offline).