I'm having trouble with my microphone.

You can adjust your microphone settings using the Options->Voice tab from the main menu while playing. You can test if your microphone is working in-game by typing "voice_loopback 1" in the console. You can get access to the console while playing by hitting the "~" key. You should be able to hear your own voice with voice_loopback set to 1.

You might also try starting the game with "+voice_dsound 0" in the command line to see if that makes a difference. Right-click on the game in your "Games" list, select "Properties", and select "Launch Options" to enter command line parameters.