Adjusting Sound In App

To adjust sound settings, head to the Extra's Menu and then scroll down.

The Master Volume Setting will adjust all sound In-App (actions, battles, upgrading and music).
The Music Volume Setting will only adjust the background music In-App, and not effect the other sounds (actions, battles etc.)

Adjusting Sound/Volume on your Device

If you've already checked the Volume/Sound settings In-App. You may need to adjust your devices volume. 
Often there is a volume button on the side, but if you are still having trouble try this handy Android Guide!