sometimes computers share an input such as line out with mic in.
open a terminal and run
as a local user
Press the TAB key to get inputs or (capture)
use the right arrow key to get to mic
press space bar to activate it...or unmute it.....settings will turn red
press page up to raise volumes
2) if no capture device is seen then press tab key to go back to outputs
one of these may need to be navigated to ....and then press the up or down arrows to change it from an output to a mic input.
I have some images and settings here