So I just dug up an old Logitech 5.1 speaker system that hasn't been used in a long time. The two rear speakers were missing, though, so I bought a pair off a friend to replace them.
I made the changes in the Control Panel and my sound card driver so that it's configured as a 5.1 system. I can test each speaker individually in the Control Panel and everything seems to work fine from there.
Also everything works fine in Crysis - no changes were needed to make it behave like it's supposed to.
In Starcraft 2, I had to manually set the sound configuration to 5.1, because it auto-detected it as a 2.1 system. Everything works fine now though.
In other applications, though - browsers, iTunes, media players, etc - it pretty much still seems to think it's a 2.1 system, as the rear speakers are always silent. Any ideas why this is/what I might need to do so that my system is fully convinced that it's a 5.1 setup?