When activating an audio element and no sound is played even though there is visual feedback, your device may be in silent mode or have the volume muted:
Check the iPad/iPhone isn't in "Silent" mode. This is controlled by a switch at the side of the device, which is positioned above the volume buttons on the right-hand side of the screen on the iPad and left-hand side of the iPhone. When you flick the switch to silent mode, a small bell with a strike through it will appear on your screen for a few seconds. When you change the mode back to sound, the bell will appear on your screen without the strikethrough.
Here is more information from the Apple support page regarding the "side switch":
Also, adjusting the volume buttons may be necessary. If you have accidentally pressed and held the bottom volume button, you might have put the iPad/iPhone on mute.
If your device is paired with a bluetooth headset or other audio device, try disconnecting from the bluetooth audio device.