External mic not working anymore on Macbook Air
I had the exact same problem on my MBP late 2015 (running Catalina 10.15.7) with a Rode Mic and a TRRS adapter. Everything was perfectly fine and during a live transmission the mic went dead. I had to quickly switch to my AirPods (awful sound compared to a external mic) to complete the transmission.
I tried ALL possible solutions documented on the web and Apple Support pages. MBP would just not recognize it anymore on the Sound properties.
I tried SMC reset, NVRAM reset, upgrade to last patch (I was one patch behind), upgrade to Big Ser (new OS).. nothing.
I decided to reinstall and restore from Time Machine, since I didn't have interest in running Bigsur yet. Voilà!!! Worked!
It seems the reinstall + Time Machine restore fixed something that was corrupted in the OS.