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 the voiceover, you can specify this while prompting the magic box
Mute the voiceover
Decrease voiceover volume to 0
Note: Muting the voiceover in specific parts of the video using the Magic Box feature is not yet available.
However, you can add pauses for chapters from the Edit Script section as a workaround, as demonstrated here.