How Do I Put Music File Over A Video In Windows Movie Maker?

Above the timeline, to the left of the "Show Storyboard or Timeline" button, there is a blue volume slider icon. Click it. Drag the button to the right as much as possible, toward the Audio/Music side. Then click the small x button and there you go!

right click on the music on the timeline, and then set its volume...done

You can easy change the setting's per clip and put it on mute so only music is playing. It was like the first thing i did.

you cant delete the audio from the video thats been locked. its like asking how you can take a song, remove the lyrics and only have the instrumental. What you can do is create an audio track under the video track and add the song to the video. Another possibility is to use video recording software on your computer; turn your volume off and record the video through the software (play muted video on computer while recording it through software application).....then you follow the same audio track process listed above....it might reduce video quality though