There are a couple things to check...
First click on the xfce4-mixer-applet...Speaker icon in the corner. Make sure that the slider for capture is turned up.
Second in a terminal type alsamixer. Tab to the capture part of that and make sure nothing is muted.
Here is a screencast that has examples of the above.http://www.opensourcebistro.com/Tutorial/VL60/02.Beginners-guide/sound/sound.htm
Finally you can make adjustment in skype itself. I have one laptop where my mic port just does not work. With this method I can use the usb mic that is included with my webcam.
Here it is...Sorry no screencast.
When you are using skype there is a little skype S at the bottom left. Click it and a menu will drop down. Choose 'Options'.
From the resulting menu choose 'Sound Devices'
There you can choose what your sound in device will be. Assuming you have a usb type webcam with microphone built in you would choose that.
You can also choose your sound out device so that the incoming voice only goes to your headphones.