May I suggest that you test the integrity of your microphone by using a sound record utility.
(ie. independent of the skype client).
If you have installed the package sox_14.0.0-5_i386 then you will have access to the rec, and play commands from within a terminal shell.
Try recording your voice using the mic with the rec command and playing it back with the play command.
Also check the mixer controls on the desktop to ensure that the mic is not muted.
Link: Ubuntu manual: http://manpages.ubuntu.com/manpages/.../man1/rec.html