I'm having this problem, too, and I've had it for about half a year.
The volume adjustment seems to work, but frankly there's always a new solution. It can be anything from adjusting the volume to restarting the computer to making sure not to run Citrix at the same time or any combination of them.
The list of inputs and outputs is sometimes entirely empty as well for no apparent reason. The list of inputs and outputs also does not update if I plug in or plug out an extra microphone or headset.
The Teams Linux client is outrageously bad... :/ I really appreciate the effort, but there's got to be some more effort put in here. These errors are common and easy to reproduce.
Pop!_OS 20.04, Ryzen 5900X, RØDE microphone, Logitech webcam microphone, Steelseries Arctis Pro Wireless, a friggin' Valve Index headset microphone - you name it. The microphone doesn't matter.