I fixed the previously mentioned time jump problem, at least well enough.
I also now have 2 (roughly) identical microphones set up, and while the initial results were frightening, they settled in and starting behaving reasonably.
My first attempt was right after I had plugged them both in, and the result was this:
There was about 90-100 millisecond skew in their readings, which since they were right next to each other, seemed very odd. It’s the kind of delta I would expect if they were 50 meters (165 ft) apart. I thought that maybe my time sync wasn’t nearly as stable as I thought it was, but I decided to give them some time to settle-in and converge, and that helped. After 10 seconds of being on, I got this result:
This was much more encouraging. The waveforms are almost synced. If I zoom in on the peaks, I can see a better estimate of their delta:
The actual time difference was about 400-500 microseconds, and at least part of that (maybe 100-200 microseconds) might be due to them being 6 inches apart. This is much closer to the kind of synchronization I’m going to need for this project.
Update: Celebration premature it turns out. I still have serious NTP sync issues that are causing the two microphones to fall out of sync. Ripped out my previous hairy math and trying to fix it now, but still having problems with the corrections oscillating wildly.