If I'm explaining the more than obvious here, I apologize in advance...

You have to make sure you have the proper audio input selected and that its level is set to a decent level. You do this by right-clicking the little speaker in the bottom right corner of your screen, select "Adjust audio Properties", "Audio" and then 'sound recording'.(this layout may differ depending on what OS you're using, but you'll figure it out)

I hope this simple fix will do it.