Normally I have my iPad on mute and just turn up the volume when required. Recently it has refused to turn up the volume.

It is not the rocker button as that works fine for other things.
It is not the speakers as it can turn on keyboard click and hear that.

Any ideas why YouTube and Real Racing 3 won’t let me add sound?

Have you disabled notifications and turned off system sounds? The wee icons in the quick access system tray thing beside wifi/bt/rotate, if that makes sense.

I normally have system sounds off but I turned them on to check the speakers were working and that was fine. Keyboard clicking for example.

Notifications is off but I didn’t think that affected volume. I will turn it on and see

There are some sound bugs in the recent OS updates. Affected a few apps I use.

Go into settings / sounds and see where the volume is set and also if the volume buttoms have been disabled