First of all, check so that the sound isn’t just switched off. Find the sound switch on the side to make sure there is no orange stripe/dot showing.
Also, sometimes kids accidentally press the volume down button or accidentally access "AirPlay" mode. Try pressing the volume up button or disable "AirPlay" mode in the control panel.
If you're using an iPad, the sound issues could also be related to the physical sound/rotation lock side switch. Here's how you can fix it:
1. Go to Settings > General > Use Side Switch To > and make sure Rotation Lock is enabled.
2. Toggle the side switch to off (the orange dot showing)
3. Open the app. Can you hear any sound? Probably not.
4. Go back to 'Use Side Switch To' in Settings > General >Use Side Switch To and change it to 'Mute'.
5. Set the side switch to on (orange dot not showing).
6. Open the app again. Can you hear any sound?
If that doesn't help, try rebooting your device. Here's how to do it:
Instructions for iPad
Instructions for iPhone