Try the following instructions to narrow down your problem.
1. Download and reinstall latest driver for your audio card.
2. If you have some third-party phone software such as Skype installed on the computer, please temporally uninstall the program as it might lock the volume level.
3. I wonder whether you changed microphone volume level through operating system tray bar or in some applications.
4. Try Start -> Control Panel -> double-click Sound icon, then select Recording tag; Select your audio device and click Properties, select Levels tag, and adjust the Microphone level. Then go to Advanced tag, and clear the "Allow applications to take exclusive control of this device" from Exclusive Mode. restart the computer and test the issue.