Well there Dinges, more info is needed like the type of sound card (so that I know your onboard mixer is correct)

and the type of platform (apple or PC, XP or vista) but I can take a shot in the dark
that you haven't set up your (play-back and recording) preferences in control panel.

Right click on the little bell(volume) on the bottom right taskbar
select (open volume controls)
select options at the top left to open it then select properties
Here you will see both play and record mixer funtions
check record and make sure your mic, line-in, or stereo mix is checked depending if you want to record internally or not.
Does this help??
Brad
B.W. Mac - recording