Question about windows 7 movie maker...?

I have a windows 7 and have downloaded movie maker on it and use it a lot. It works fine, but I don't know how I can add sound to my videos (besides music, which I know how to add). I don't have like a voice recorder or anything, but is there a way I can say something into my video camera, and make it so that it's only recording the sound and not picture? (So basically I could have the screen showing a picture or video and have me talking in the backround). Thanks.