I've been playing Dark Souls since it came out, but just started doing it with 5.1 surround sound (Onkyo TX-NR717 receiver and Energy Take Classic 5.1 speakers) for the past few days. Everything was great until I reached Anor Londo and started fighting O & S. The battle music, not the sound effects, come out of all but one speaker, the back left one. I completely lowered all the in-game volumes except Music and confirmed that the [awesome] battle music does not come out of the Rear Left speaker. Sound effects work fine, but not this particular music.
This happened in the PS3 and XBOX 360 versions. Other games and parts of the game seem to work fine, but I just want to know if anyone else has experienced this. It could be my setup and I just didn't notice it in other games before.