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Apple changed the functionality of the button above the volume controls on the iPad to work as a mute button and a screen lock button. Make sure this mute button is turned off when you use the app and that should fix any sound issues. To do this, simply toggle the switch the other direction to see if it activates the sound.
If it's still not fixed, there is a control in the settings section of your iPad that allows for changes to its functionality.
Go to Settings - General - Make sure the mute button is checked as below:
iPad Air 2 - Adjust from Control Center
Unlike previous iPad models, iPad Air 2 doesn't have a side switch. Learn how to turn on Mute and Orientation Lock using the Control Center on your iPad Air 2.
On your iPad Air 2, you'll find Mute and Rotation Lock in the Control Center. Swipe up from the bottom of the screen to reveal the Control Center.