External mics are not working

When I use any type of external microphone with Zoom (Rode Wireless Go, Heydey microphone, etc) there is this awful buzzing sound and nothing works.  The only mic that works is my built in microphone.  I use a MacBook Air 10.13, and yes - I have updated the app, the cords/plugs are all in the right places, nothing is muted, etc.  

This started happening with my Rode but now no external mic works.  

I use Zoom for fitness classes (with the Rode) and for corporate presentations (with the Heydey), but now neither is working.

Any ideas?