Assuming all the drivers and other things are in place.
Please look at these links for that
Debugging Sound Problems
In case all look ok do this.
a) Open terminal
b) type 'alsamixer'
c) Use left right arrow keys to select and up and down to adjust.
d) Adjust, Front Mic, Mic Boost and Input Source.
e) Select tab and adjust Front Mic, Mic Boost and Capture.
f) Press esc to quit.
Try skype or other recording tools to test.
Gud Luck.
More Info here
How TO: Getting Microphone to work
