Ok, I got my sound to work.
I'm not exactly sure WHAT I did to fix it, but I found this: http://nixos.org/wiki/Audio_HOWTO
I followed that - most of which was already configured. But it still wouldn't work until the section where you check your system settings and specify the device you want to use in Phonon. Before, I would select this device and test - and it wouldn't work. Now, it magically does.