http://community.skype.com/t5/Windows-desktop-client/Microphone-not-working/td-p/234
"On the Skype menu, go to Tools > Options > Audio settings. Is your Logitech webcam showing in the Microphone pull down window? If not, can you select it with the pull down arrow?
When you talk, does the green volume indicator on deflect to the right?"
Does any of that help?