Justin Mullens, Sound Designer and Composer Extraordinaire, is the brain behind the excellent music in Boss Rush. He has thoughtfully sent over some of the music, in long-format MP3 files designed for listening. (As opposed to, say, looping in-game.)
What’s neat about this is that the actual game only contained TWO pieces of music. The other two are pieces that never made it into the game for various reasons, but were composed during the same period, with the same sort of mood in mind.
Have a listen!