Just to chime back in for the sake of @phipck, I never was able to solve the problem. I finally just worked around it by plugging my headphones into my external (amplified) speakers, then just adjusted the volume using the speakers' volume control. I agree with @shooternz - it's likely a problem with the sound card hardware or (more likely) the Asus Xonar drivers.
As an aside, a related problem I encountered at the same time (and can still replicate) is if I run PPro CS6 and AE CS6 simultaneously and then alt-tab to switch between them, full-blast pink noise static is output from the sound card. Again, this is regardless of the system volume setting (and regardless of whether I'm actually trying to actually play back any video/audio). To fix this problem I have to exit PPro and AE and just run one at a time.
-Chris