It is not needed to install alsamixer. The problem is, that Skype doesn't know where to listen and there is no option how to say in Skype environment directly. Install PulseAudio Volume Control from Ubuntu Software Center or via Synaptic.
Start Skype and choose Options from blue S down. Select Sound Services. Check if both three options (Microphone, Speakers, Ringing) has setted PulseAudio Server (local).
Uncheck Allow Skype to automatically... to prevent too silent sound.
Click button Open PulseAudio Volume Control, it should be visible down (if not, open it clasically from menu).
In opened PulseAudio Volume Control application click tab Input devices and check, if microphone is functioning (it is visible on the horizontal scale).
Than click tab Recording. The page should be empty (no another device is recording).
In Skype call Echo/Sound test service, or call somebody, who is able to endure on line. This is only that way, how to set up microphone! During the session in Volume Control is visible what is recording. Push button in upper right corner and choose your web camera microphone input from list.
In next session Skype should remember those settings, so it is not needed to do this setting always. I suppose, that it is stored in
~/.Skype directory for further session.
Tested on Ubuntu x86-64 13.10, 14.04, 15.04, 15.10, 16.04 Skype 220.127.116.11, 18.104.22.168.