As suspected, it was the cable bunching up near the antenna which destroyed the last recording. Further proved the noise went away if the txpower was reduced to 0dbm. It was at 31dbm during the fateful day & there was a lot of confidence a commercially produced sound chip was immune to it.
After much procrastination, it was decided the best option was a jumper to lower the wifi power & a flag on the web app showing the current setting. WIFI actually still works marginally at 0dbm, but you need 31dbm to transfer audio files & there's no way to set it without a physical interface. Could have made it only lower the power during recording, but it would have been impossible to monitor without recording.
A similar thing should be done to the 4 channel recorder, after some more procrastination. Of course, this doesn't defeat interference from the phone.