How to change an MP4 file to a Music file on Windows 8?

Ok so lately i have been recording Minecraft Pvp videos and uploading them to youtube… I upload them as a montage and i use Movie maker to edit (me not being very good at editing) and it worked until i reset my computer… So first i ut the gameplay in then i put the music in… But because it's not a (Music) file with Windows 8 it just overrides the video with that song instead of putting the song in with the gameplay.