iPad internal volume disabled?
I had this problem several times before I have to turn off the iPad and reset it to be able to control the volume with the slide up controls. Not sure the exact name for that but it’s when you slide your finger up and get the controls for volume ,brightness etc.
..you can still controll volume with the side buttons but with my case they don’t work.
I think this happens when I plug my iPad into an amplifier then open the app.