Sound notifications no longer working on ipad air

Hi

I have an IPad Air. I have not changed settings but I am no longer receiving sound notifications on locked screen. YouTube, Facebook, Messenger etc give the banner alert but no sound.

I also no longer get the camera shutter sound when I take a picture.

Having checked all the settings, hard reboot etc I reset to factory settings but sound alerts have not returned.

The speakers are ok as I can watch videos etc.

I asked someone to check as well but they cannot find the reason.

thank you