Make sure that it is enabled. Right click on the speaker icon in the system
tray and choose Sound Preferences. Click on the Input Tab and then on the
Down arrow and choose Microphone. Make sure that is not muted and the slider
is not slid to off.

If it still does not work you might have to remove PulseAudio (see:
https://help.ubuntu.com/community/Skype and the Troubleshooting section).