I wasn't aware of this until recently, but there are two sets of volume properties on your standard Windows volume control (the little yellow speaker in your system tray) - one is called "Playback", and is the set displayed by default. To access "Recording", you need to:

1. Double-click the yellow speaker icon.
2. Select "Options"
3. Select "Properties"
4. Under "Adjust volume for...", select the "Recording" radio button.
5. Adjust your volume levels and selected/muted inputs as required.

You can have both "Playback" and "Recording" open at the same time by following steps 1-4 above and then repeating step 1.

This may solve your problem, if, as you said, you're getting some kind of signal, but not a very loud one. Hope that helps, or at least informs.