i have a camera which has a microphone, but its not working. Why?

There are two possible problems
1) your microphone simply isn't working
2) Skype can't see your microphone, even though it is working.

The first case is easy to test -- install the program "audacity", then see if you can record yourself. If you can, your microphone is working, and the problem is problem 2.

As for the second case, Skype sometimes has trouble picking the microphone feed from other audio inputs (such as line-in). You can follow the troubleshooting guide here:

https://help.ubuntu.com/community/SkypeTroubleshooting#Selecting%20Microphone%20%28input%20device%29

Run "gnome-volume-control" in a terminal.
Check the settings in "Input" tab.

on the desktop upper panel on the right side there is the sound applet

click on it , a window will open , click on Sound Preferences ... option

a window called Sound Preferences will open that has 5 tabs ; click on Input tab

on the lower panel there should be some entry/ies with radio buttons in front

pls list here ALL entries AND, v important, which entry has the radio button enabled/checked.

for your info : if you want to use camera mike the radio button OF CAMERA DEVICE MUST BE ENABLED otherwise ubuntu will use the first entry that is usually the input interface on the mb or some other sound device present.