Please follow the outlined steps:

1. Your iPad has a mute button right next to the volume controller along the side of your device. Please make sure that your iPad is not muted (in the mute position, you see a little red dot next to the mute button)
2. Except iPad Air 2, you can choose under ‘Settings’, ‘General’ the side switch to either function as a ‘Lock Rotation’ or as a ‘Mute’ button. If you had your iPad muted and then change the side switch to function as a lock, you will not hear any game sounds. Please switch the device back to ‘Mute’, then open the app and check that the sounds are working :)
3. Some apps override the mute button so you can still hear sounds. This app only plays sounds when the device is un-muted or when you wear headphones while the device is muted.