Very few camcorders have a line in for audio, so it gets hard for regular folk to do high quality audio/video demos.

But there a little trick you can do...

If your video camera is digital, and you have a little audio digital recorder (a Zoom, or similar), simply record the demo to the pocket recorder while you are shooting the video.

Then, most video editing software allows you to 'import' an audio track and line it up with the video. Line up your new audio with the audio from the camera (slide it around until there is no flamming), then discard the camera's audio track...

Voilá! A video with 'pro' quality sound.