Well, the plug is definitely in all the way - and I've tried it in both my line out and my case headphone jack. I know it's not my speakers, since I've tried headphones on both my volume control remote and straight from the headphone jack on my case. I tried restoring to an earlier restore point with no change. I found the setup file in my driver extraction and went through that process, but no change.
The only thing that I can think of is that my motherboard's audio controllers were damaged in a power surge, since my power went out this morning. I'd been having other issues (including the launching of startup repair, though I never before had to resort to system restore) before the surge this morning but this audio problem is new and incredibly annoying. I'm thinking that I need to replace my motherboard, since it's got one of the faulty Sandy Bridge chipsets and I've already been experiencing disk drive (SATA port 4) issues anyway. I might try a reformat before then, but I'll probably wind up putting this faulty board in a new lower-end build with a cheap sound card.