Teams is confusing Mic and Speakers on Teams (Mac OS Big Sur).

Hi, Teams on Mac OS Big Sur seems to be confusing the Mic and Speakers sometimes, now this usually works fine - but sometimes it reverts back and Teams will have no sound output, and will sometimes crash or freeze completely:  

Any idea what is happening or how I can fix it? I've tried reinstalling Teams several times, and this is persistent across user accounts.  I'm using a USB sound card, and a Dell D6000 Thunderbolt Dock.

Any help would be much appreciated! Cheers.