How is the microphone connected or is it a built in one?
Suggest you open alsamixer in terminal to investigate all your sound input channels.
In alsamixer check that none of your sound channels are showing muted "M". If any are, pan across to it and unmute by typing M.
Also in alsamixer, check Auto-Mute status, and change it if set.
In my experience, you may have to do a bit of trial and error in alsamixer to get sound channels working properly. You could post a screenshot of your alsamixer, which may help us find the mic problem.