To mute the video in WMM (Windows Movie Maker)
You must be in the Time-line mode of WMM
> Click the + button beside the word Video on the video track
> Right-click the Audio track of this menu
> Click Mute from the menu
Voila! No more audio on the video! You can also use this menu to lower the video volume level, fade the audio in/out, etc. Just remember, the + sign beside the word Video in the Time-line view opens up the menu options specifically for the video in your project.