1. download and install drivers for the soundcard. (reboot and test after each step)
2. install some system codecs such as the k-lite mega codec pack from here
http://fileforum.betanews.com/detail/KLite-Mega-Codec-Pack/1080441198/13. go through and check options in control panel-->sounds, go through all the options and ensure that nothing is muted, and that you also have a device listed under the audio tab and "sound recording" section (change the item if necessary and test again. Check the volume too.
4. go to start then run and type dxdiag go through the sound tests (yes strangely this has worked for me in the past)
4. test the microphone with different software, such as windows sound recorder, start, programs accessories, entertainment, sound recorder. Or if you have skype installed, do the test call.
5. Does your computer have onboard jack for the microphone (not the add in card)? test this too.