The StarCraft 2 collector's edition includes a soundtrack CD, which you can also buy independently (here's one offered in Amazon, and Ivo posted a link to iTunes where it's much cheaper).
In addition, you might be interested in this post on GosuGamers, which points to a series of YouTube videos with the different tracks.
Keep in mind that if you do find some way to extract the music from the game and transcode it to some convenient format, I'm guessing sharing it could be considered a copyright violation.