Note: From my experience, all sound issues come from the side switch that can be used to mute sound or lock rotation.
This is a frequent issue due to the strange way this switch works.
Some of our apps use the iPad side switch (or iPhone silent switch) to turn sound off, so the issue should be related to this switch.

So first check if this switch is not enabled (the switch is located next to the volume buttons, and the orange should not be showing).
You can also check the volume buttons.
Note that the side switch does not mute Media Playback but only alerts, notifications, sound effects, and app audio - so your may think that this is the app that has an issue but it is only related to the side switch.

You can also check this Apple support page :
iPad: Understanding the side switch

We've also seen a bug where the sound will stay off if you change the Side Switch to "Lock Rotation" while the iPad is Muted:
Here's the solution to this issue:
• Make sure the orange dot is not showing on the Side Switch on the upper-right corner of the iPad.
• Go to Settings > General > Use Side Switch to: > Check Mute
• Open the app, and the sound should now be working (if not, try to use the volume button)
• (Optional) Now if you want to change the Settings to “Use Side Switch to: Lock Rotation,” you can. With the Lock Rotation setting selected, you can change the Side Switch to show the orange dot, and the app will now still have sound (but do the changes when iPad is not muted)


For other issues, you may try to restart the app, or reinstall the app (reinstall is free, you won’t pay the app again). You can also check this Apple Support page : http://support.apple.com/kb/TS1702

Restart the application
•Press the Home button to return to the Home screen.
•Double-tap the Home button to display recent apps.
•Tap and hold the affected app until the red minus appears.
•Tap the red minus to quit the app.
•Press the Home button, then restart the app

Reinstall the application
•Touch and hold any application icon on the Home Screen until the icons start to wiggle.
•Tap the "x" in the corner of the application you want to delete.
•Tap Delete to remove the application and all of its data from your device.
•Press the Home button.
•Go to the App Store.
•Search for the application and then download it again.

If you can't solve your issue, don’t hesitate to send a mail to contact@onetreehillstudio.com