Google Assistant updates.
Once I had my Google Assistant running Raspbian Buster, and verified that it could not only answer spoken questions, but also speak text sent to it by other computers, I made a bad mistake.
As a rule, it's a good idea to keep a Raspberry Pi's operating system up to date. Not on this system, it turns out!
I found that doing so stops the microphone working. Other people have also found this, according to the internet, dating back to 2018.
It was simple enough to fix, once I knew that the update was the cause of the trouble. I just flashed the micro SD card again, and copied my configuration files back onto it. Just like that, it was all working again. The files you need to copy back on are assistant.json client_secrets.json and credentials.json, if you were wondering.
In order not to let the same unpleasantness happen again, I disabled the update programs apt and apt-get, using the commands
sudo chmod a-x /usr/bin/apt
sudo chmod a-x /usr/bin/apt-get