Have you checked that Skype is using the microphone you want to use. Many notebooks/desktops have build in mics, or other devices plugged in/built in that the computer recognizes as mics... So... You could try doing like this; and give me results:
1. Right-click on the
icon. Select "Recording Devices".
What devices do you see here?
Also try switching (if you know which mic is the one you want to use) the default device.
2. Open Skype. Click on "Tools", then "Options". Next, click on "Audio Settings". Now test all the devices.
Tell me how you get on!
It could also be hardware issues... You know, the mic could be broken!