Is your sound card installed properly?
If it is, go to the control panel and open the Sound and Audio Devices icon.
Click on the Audio tab and make shure the default device under the sound recording section is set to the sound device you are using.
Click on Volume tab, and under the Device volume tab, select Advanced.
Select options => properties and make shure there is a check mark next to Microphone.
Click on OK, you should now see Microphone listed in your volume bar.
Last step. Uncheck the mute button under the microphone slider.
Your microphone should now be working.