Most often, this is because the iPad is muted. Apple has a half-hearted attitude towards muting. Apps have the option to ignore the mute setting, and many do - giving the false impression that muting is off. A further complication is that in iPads with a side-switch, the side switch can be used either as a mute switch or an orientation lock. If the iPad was muted when the switch's function was changed to orientation-lock, the iPad will remain muted regardless of the switch position.
First, check if your iPad is muted - either at the side switch, or if you have an iPad Pro, iPad Air 2 or iPad Mini 4, the mute switch is in the Control Center. https://support.apple.com/en-au/HT203121.)
If you have a side switch and it makes no difference to the muting, check the side switch function as follows...
- Navigate to Settings on your iPad.
- Tap on General and scroll down to the “Use Side Switch to” box and select the mute option.
- Toggle the side switch to the off position. The app should now have sound.
- To lock the rotation again, just go back to settings to switch what the side switch controls.