
из http://www.vcdhelp.com/mjpeg.htm
If there is no audio check that line in is not mute.(Double-click on the speaker icon in the tray and go to Options -> Properties -> Adjust volume for recording. Then you should be able to alter the volume of the line-in port.)
If it still doesn't work try to restart VirtualDub sometimes.
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Fortuna non penis, in manus non respius