I tried to make my own offline video clip. so I downloaded some opensource screen capture application. and then I captrued video clip however sound was not there.

Long time I haven't set up my windows control panel.

so I need to goolgling again.

That's what I found from Ken's Blog.

http://cow.neondragon.net/index.php/2141-Recording-Sound-Output-In-Windows

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1. Select your sound properties through the control panel or the sound icon in your system tray.
2. Click on the properties tab.
3. Check the recording box.
4. Make sure Wave Out Mix is selected (It’s sometimes called stereo mix, or mono mix).
5. Now you should be able to set Wave Out as your default for recordings made by Audacity or other audio recording programs.