Title says it all. I'm using realtek onboard audio. I don't know if this has anything to do with it... but at one point I did switch to 2.1, and I think during that period I launched steam and it updated to the latest version. I don't recall launching NS2 though...
Anyway I've switched back to 5.1, and all my other games are doing fine with 5.1. I tried switching back and forth between 2.1 and 5.1 again. I tried switching from analog output to digital. I tried deleting the game from steam and redownloading it. I tried deleting the Natural Selection folder with the options.xml in it. None of these worked.