If you lose sounds for keyboard clicks, games or other apps, email notifications and other notifications, system sounds have been muted.
System sounds can be muted and controlled two different ways. The screen lock rotation can be controlled in the same manner as well.
Settings>General>Use Side Switch to: Mute System sounds. If this option is selected, the switch on the side of the iPad above the volume rocker will mute system sounds.
If you choose Lock Screen Rotation, then the switch locks the screen. If the screen is locked, you will see a lock icon in the upper right corner next to the battery indicator gauge.
If you have the side switch set to lock screen rotation then the system sound control is in the task bar. Double tap the home button and in the task bar at the bottom, swipe all the way to the right. The speaker icon is all the way to the left. Tap on it and system sounds will return.
If you have the side switch set to mute system sounds, then the screen lock rotation can be accessed via the task bar in the same manner as described above.
This support article from Apple explains how the side switch works.
Thanks - I feel like a complete idiot. Sometimes I use the side ROCKER to mute the sound, and didn't even realize that there is a side SWITCH. Interesting to learn that the switch will mute some sounds, but not all sounds like music. In any event, I'll admit my stupid mistake here in case it helps someone else!
Don't be so tough on yourself. Many users aren't aware of how this works.
However, I have seen way too many users that insist that they cannot unlock the screen rotation AND have the system sounds turned on at the same time, which is obviously ridiculous - but they continue to insist that it cannot be done because they do not understand how to do it.