Why can't I get any sound out of my camera?

Recently my mom bought a camera. Its an Olympus T-100. It has 12 mega pixels. I really love the quality of this camera and since I make videos on youtube I decided to try using the new camera for videos. But unfortunately there is not sound even when i connect it to the computer :(
I am confused with whats happening because before. I recorded my first video with it and it had sound. but then the second or third video there was no sound. I have read the manual script and everything… I am also pretty good with anything to do with cameras so I do not think that it has anything to do with me putting it on the wrong settings…
Any helps?