Well, replacing the mic with music is easy. You can change that in the audio settings panel (Options -> Properties -> Record -> OK). Then you put the tick on the input source you want.
The video is probably harder or you'll need some kind of convertor thingie...
When I still had my TV card for receiving cable TV on my computer, I was able to broadcast channels on MSN Webcam... it's just a video input, so I was able to choose between the webcam and the cable card... that was fun.
But really using video files you have on your computer, I don't know about that.
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