If you want your video to not have any background music, you can select the 'Mute' track. You can do this on the 'Storyboard' screen and the 'Advanced Editing' screen. Here are the steps:
Step 1: Go to the 'Music' tab on the left panel.
Step 2: Select the category 'Mute.'
Step 3: Select the track 'Mute' under it.
Please note that this means your video has a music track, it is just silent.
We are working on having more advanced music controls.