I ran some Google/forum searches, and it seems many people are experiencing this issue. The only official response I've seen (from Gstaff) was to update audio drivers, but it's not audio drivers, because many complainants' drivers are fully up to date. Also, this bug apparently appeared in version 1.4, and wasn't present before then.
I've tried several things including installing the Unofficial Skyrim Patch and tweaking the sound/mixer settings and enhancements, but nothing has worked. I haven't found any solutions elsewhere on the Internet, either; no one seems to know.
As an aside, I'm pretty angry that major bugs in Skyrim go unfixed for months at a time on PC. This NPC voice problem has been unsolved since February, from what I've been reading. Bugfix patches from Bethesda are also very few and far between, if Steam's Skyrim update news is any indication.