iPad
Most sound issues are caused by the iPad’s finicky settings and mute function. Make sure you follow the steps outlined here carefully to find out if your iPad could be muted: How to tell if your iPad is muted

Also try visiting Settings > General > Use Side Switch to: and switching that back and forth a few times. Sometimes the mute just gets stuck and it needs to be shaken loose.

iPhone
Make sure your phone is not muted. It helps to quit the app, flip the mute switch so that it is not silenced, turn the volume up and then try launching the app again.

After going through those steps thoroughly it helps to try restarting or reinstalling To Restart hold the top button down until the red slider appears to turn it off. After you’ve turned it off hold the button again to turn it back on. To reinstall the app first delete the app by holding down on the icon then click the X that appears, then go to the App Store and download it again. You will never be charged twice for an app you’ve purchased.