Also, check the two following areas.
1. The Mute Switch – Most iPads/iPhones have a switch on the side of the device just above the volume control. This default behavior of this switch is to mute your sound. Sometimes it can be turned on by accident when taking the device in and out of a case or simply by being caught on something. Try sliding the mute switch in the opposite direction to see if sound is restored. Often this is the root cause of no sound on an iPad/iPhone so it is always worth trying this first.
2. Control Center – Although the side switch will mute your audio it can be used for another function. Soto double check that your volume is not muted launch the Control Center by swiping up from the bottom of your screen. Look for a bell with a line through it. It will be one of six icons in the middle of the Control Center. If the bell is white the off position, tap on it to turn it black, then try launching an app with sound to see if your audio has returned.
Then double check all your sounds on both the phone and app is all the way up.
Here is also a video that you can try to see if this helps in explaining the above information.