Splitting video from AVI file

I'm working on a school project and I need to narrate over some .AVI files we made. I don't know how to mute these files or to separate the audio from them. Can anyone help me with this? Is there a local program I can use or must I download something else?

EDIT: Figured out how to do it with my old copy of VirtualDub that I used for PSX emulation.