First, go to Start, type dxdiag and open the utility. After that, click the Sound tab, check your sound card model and download and install the latest drivers available from the developer's web page. After the installation process is complete, right-click on the sound icon from System Tray and select Playback devices. On both Playback and Recording tabs, right-click on an empty space and select Show Disabled Devices. After that, right-click on the disabled devices and choose Properties. Select Use this device (enable) as Device usage and click OK. Next, from the Recording tab, select again the Microphone device and click the Set Default button next to Properties. Also, make sure that your microphone is plugged in the red/pink jack.
If the problem is caused by your webcam device, press Windows + R key combination, type devmgmt.msc and click OK. Expand the Universal Serial Bus controllers list, right click on the webcam driver and select Uninstall. Reboot your computer and download and install a compatible driver using Google search engine.