Enter the Setup screen of the application (press the Settings button that appears on the main screen at the bottom right) and then check if the sound option is turned ON. If this does not correct the problem, try the next option.
Check the mute button on your device. If the mute option is on, turned it off.
Delete the application and reinstall it. Do not worry about buying the app again, the payment is only made once. To delete it, just press the application icon until it starts to “shake”, and then reinstall it from the AppStore again. Be sure to do this with the same account that it was originally purchased with. Next, restore any purchases in the app settings if the app is the in-app version.