If you haven't published your video yet, tap Edit in the top right corner.
Tap Change Music Selection and Music Mix Volume.
Select a music genre.
Select a music track.
Use the slider to adjust the sound mix.
Tap Save to save the new sound levels.
Tap Save again to generate a new preview.
If you're video has already been published, find it under My Account and tap on the ellipsis icon.
From here, follow the same steps to adjust the sound levels and upload a new copy of your video.