There is a special soft-mute “feature” on the iPad that, when turned on will turn sound off on some apps (eg. Puzzingo and Wonster Words), while still allow other apps to play sound (eg. Pandora, Apple Music).

So you have already turned up the volume on your iPad, turned off the mute slider, but still can’t hear the sound, this could be the reason:

• Swipe up from the bottom edge of the iPad. Make sure you do this from the very edge of the screen. This will open up the Control Center.
• Find the “soft-mute” button, which should look like a Bell. Tap on the bell to un/mute your iPad.

That’s it!