From the My Videos screen find the video for which you want to change the sound levels and tap the Gear icon.
Then tap the Edit button.
Tap Change Sound Volume from Videos Selected in Step One.
Locate the shot that has the sound volume to be changed, then tap on it.
Adjust the mix for that shot by moving the slider to the left or right, then tap save.
Once you’re done making changes to the sound levels, tap Back.
If you’re done making changes to this video, tap Save.
Then tap Save again to give your video an updated title, then tap Save to let it process and upload.