I play on a laptop. My mic used to work on source games, but i don't know what happened. I stopped playing for a while and i just realized now that my mic doesn't work. It works in everything else other than source games. I tried reinstalling gmod. I tried switching between chromium, x64, and default versions. I went to my sound settings in windows and made sure that my mic was the default device.
I have a focusrite now, but it was not working since before i had it. I tried doing the same steps for the default laptop mic and the focusrite. Both dont work. I have no other audio devices...
Take a look at this threat as well he had a similar problem.
I've had this problem as well myself. What I did was set everything to default in Steam and gmod itself, then validated my Steam files, then rejoined again, and after that, it solved my problem.
If that doesn't fix it, please send a screenshot of your input devices in Windows Sound Settings and wherever else you are using it. Try updating audio drivers too.