Sound Volume Icon Keeps Popping Up

I have a brand new M1 Mac mini, just started it up for the first time two days ago. Everything was fine until today, when the sound volume icon, the one that appears in the center of the screen when you adjust the volume, started appearing. It started popping up about every 10 seconds, then fade out after about a second. Now, it's continuous, as soon as it fades out, it pops up again. Also, sometimes when it appears, it adjusts the volume up or down a tick or two. Other times, there's no change. This is making the computer unusable.

It doesn't appear to be a stuck key on the new Magic Keyboard (in use for two days). The only external devices connected are the display in the HDMI port, and two drives in the two USB ports. The only thing that's changed in the past two days since setting up the computer, is the installation of some Adobe and Microsoft applications.

Any ideas how to fix this? Could it have something to do with something migrated over from the old system? If so, why would it take two days to start happening?