You are probably seeing this because you wanted to adjust the volume of your background video/background music using both at the same time.
Kindly head over to this as Video Settings>Media>Volume:
Set volume for background music in this scene - Adjusts the external music applied
Set volume for media in this scene - Volume of media's own sound
Set volume for voiceover in this scene - Volume of the voiceover
To come back to the question, if you want to mute the volume of your video and apply a music over it, keep "Set volume for media in this scene" as 0 and "Set volume for background music in this scene "100".