See if this works for you. This is how I have my iPad set up.
Go to Settings>General>Use Side Switch to>Lock rotation. Flip the side switch the other way, if you see the lock icon in the upper right corner of the iPad screen, you have successfully set the iPad to lock screen rotation in the settings. Flip the switch the other way to unlock the rotation.
Now double tap the home button and swipe all the way to the right, look at the speaker icon all the way to the left. If it has a line through it - tap on it to unmute system sounds. Tap above the task bar to go back to the home screen.
Now your screen should be unlocked and the apps should have sounds.