Other user in server volume turning down in the middle of their sentence then recovering

this has been happening for months. They and no one else is a priority speaker. They are in the middle of a conversation and they just get so quiet no one can hear them. All of us in the server are witnesses to it, so it’s not just on one of our ends.

Any idea what the heck is going on?

Please have a look at Problem with Automated Gain Control (AGC).
Maybe you will get more help in this thread.