Figured it out!!
Short Version: There was a rodent nest. The timing with the stereo installation was apparently just coincidental -- though it's possible there was some debris in there as early as May, and the install knocked something loose that partially blocked the vent(s).
Longer Version: Last month I took my Z to the shop for her 45k-mile service, mentioned the airflow thing, and asked them to check it out. The guy said there was no blockage they could see just by looking in the vents, and we were both reluctant to have them pull the dash apart, so I just chalked it up to early dementia. Then, around the middle of last week, a horrible noise and vibration started in the dashboard whenever I turned the fan on. I park under a tree, so I figured a big acorn or something had dropped in there and gotten stuck in a bad spot. I brought her in yesterday, and -- in addition to an impressive acorn collection -- they found evidence of a rodent nest. I told the guy it was nice to know that I
wasn't crazy, after all (at least this time)! They cleaned everything out, replaced the cabin air filter, and replaced two fuses that had been damaged (there was evidence of some chewing under the hood, but luckily all of the wiring is intact).
Everything's back to normal now.
I don't think there's anything I can really do to keep this from happening again, short of moving to a place with a garage. I could put a bait station next to the house, but there are so many critters in the neighborhood that I can't imagine that being very effective. There is no other place for me to park. It's the first time this has ever happened to a car of mine, though I'd certainly heard of it happening to other people.