When you open Tandem first time, you’ll be prompted to give permissions to you Microphone. If you denied you'll have to go through System Preferences
Open System Preferences, then click on Security & Privacy, then click on the "Privacy" Tab.
There, make sure Tandem is checked.
I can't find Tandem in Microphone tab
Open app the Terminal app. To do that, you can go to your Applications folder, or use Spotlight and search for Terminal.
Then, paste the following -> tccutil reset microphone
Press enter to run the command
Open Tandem again