Asumming that your useing "Fraps' you have to unplug your mic, and in fraps turn on the sound recording.
what i do is film with no sound and add in what sounds i want in win movie maker, then overdub all of it with music. this will drown out most of the sound, so after you turn it into a movie, reload it in wmm and add back in the sounds you want to be heard over the music, you can change the volume of each sound file you add, you can also change the volume of any sound, altho you have to be careful with the sound on the movie you made other wise the level will be up and down thru the film.
you just have to experiment until you find what works best for you.
Don