Step 1: Open the sound controls by right-clicking on the loudspeaker symbol in your task area, then select "Open Volume Control"

Step 2: This window appears (or one looking similar), select options, make sure "Advanced" is checked, then select PROPERTIES from that menu....

Step 3: Switch from Playback to RECORDING properties, and also make sure that "Microphone" is listed and checked in the list below....

Step 4: The window will have changed to RECORDING controls, which means all changes here are NOT audible to YOU directly. Now check Microphone as recording device if not yet checked, turn up its volume and click on the Advanced button....

Step 5: Check the Microphone boost if not yet done...

And of course, remember to SCREAM as Carnage told you, in case the above doesn't help.
Enjoy