This game popped to my mind after listening to the Frozen Synapse soundtrack, as I think it had quite similar music (but possibly just one track).
You swing a ball with a Worms-like ninja rope through dark caves in 2D. If I remember right you you control the ball with WASD and shoot/detach your rope with the spacebar. Hitting the walls wasn't instant game over, but instead you could hit them maybe 3 or 4 times. It also allowed the use of custom music by placing the mp3 files in a certain folder.
I think it was released either in 2005 or 2006, because I remember listening to Poets of the Fall's first album while playing it.
Had no luck with google, so I'm hoping that perhaps someone here might remember it.