The audio output might be set to another device (ex: HDMI monitor instead of your AirPods). To confirm what audio output device is being used by the iPad, and to change the audio output to another device, try the following:
1. From the Home Screen in iOS, swipe down twice to launch the Control Center window
2. Press on the AirPlay icon (top right) to expand the audio window
3. From here, you can select the desired audio output device as well as test that the audio is working as intended