Oh, wow, was just about to post the same. ONLY on Zoom. Calls start fine, and then after some time, sound starts getting underwater-y, and worse and worse. Other participants also say my sound gets quite bad.
I disconnect and use speakers and MacBook Air mic, and all is fine, so it's not the Zoom connection.
Very new problem; last version or two, or perhaps the AirPods firmware update. This is really frustrating.