First make sure the volume is not muted on your device, and the volume slider under the "parents" menu of your Dr. Panda game is not set to "off".
If that does not solve the problem it could be due to the side switch on your iPad. If you mute the iPad using the side switch, then later go to settings and change the slide button to lock rotation, the apps muted with the switch will stay muted even if your volume is up.
To fix this, go to the settings menu and make the side switch “mute” again. Then use it to mute and unmute the iPad. After doing that you should be good to go!