Your iPad will trigger an audible alert when notifications are received by the device. This can be useful in certain environments where staff are not always looking at the device, for example when an order comes into the Kitchen. However, in some cases it may not be appropriate for your device to make alert sounds.
To put the iPad into silent mode:
Swipe up from the bottom of the screen to reveal the quick settings.
Tap the Bell icon.
This will toggle between silent mode (on/off).