a) The first option is that you enter the app Setup screen (press the Settings button that appears on the main screen at the bottom right) and then check if the sound option are turn ON. If this does not correct the problem, try the next option.
b) Check if the mute button of the iPhone is activated, disable it. Check if the problem was corrected.
c) If the problem continues, delete the application and reinstall it. Do not worry about the payment, you will not be charged again, payment is made only once. Be sure to install it with the same Apple account that if was purchased from.