You can mute the video to remove both music and voiceover from your video by prompting the magic box to do so:
Mute the video
If you only want to remove either the music or the voiceover, you can specify this while prompting the magic box!
Mute the background music
Mute the voiceover
Decrease music/voiceover volume to 0
Note: Muting voiceover in parts of the video is not possible yet - we are working on bringing this soon.
However, you can mute the music for certain chapters from the Edit Music section as shown here.