Having trouble getting any sound in the Ooka Island app, while other apps like YouTube seem to be working fine? Check to make sure that the iOS device you are using isn't on mute.
Swipe from the bottom of the iOS Device up to bring up the Control Centre. You will see 5 buttons, ensure that the button on the right is not white (should be grey).
If you look on your iPad/iPhone right above the volume buttons there will be another button that you can flip on and off. Simply flip that switch so that the red color is no longer visible. This should take your device off vibrate mode so that you have full sound in the Ooka Island app.