Music, is the background music for the whole video(all scenes combined).
Media, is video's own music track/narration.
Voice-over, to be simply put, is narration of a video, for a specified scene.
I have added a voice-over in my voice to the video in a scene about 'what is the procedure of making cupcakes':
For those scenes whose voice-over and music track would clash together, volume could be adjusted this way:
Navigate to Media>Media Volume
Set the volume for your scene- Sets the volume of your Background Music
Set the volume for your media- Sets the volume for video's own sound
Set the volume for your voiceover- Sets the volume for the volume of the Voiceover
You could adjust the volume by adjusting the pointer on the bar for each.