First of all check that the mute button on the side of your device is not activated and that your volume is also turned up while the App is open..
iOS7 on the iPad has a separate mute button that affects certain types of Apps.
If you open the lock screen by sliding up from the bottom you should see an icon for a bell that may have a line through it. Tap this to toggle the mute and then try the app again.
See the attached image.