Make sure your Microphone is plugged in the Microphone inlet and not the Headphones inlet
On Combat Arms Settings, Go to your Audio settings and select your Microphone and adjust your Settings
Make sure your Computer Even Notices that your Microphone is Plugged in by going to
Control Panel>Hardware and Sound>Sound>and Click on the "Recording" Tab on the top , If it says your Microphone is On then Your Problem will Most likely be Combat Arms Audio Settings;
If it says Nothing I plugged in then Something Is wrong with your Computer and Needs to be checked out .
Make sure you Try Both Microphone Inlets, There is one in the Front of your Computer Tower and in the Back . (For Most Computers)
Also, Make sure your Team Chat is Enabled (Press F8 on your Keyboard)