It sure sounds like you have system sounds muted, but if you reboot and the sounds come back, that doesn't make sense either. This is my standard explanation of how systems sounds works and can be controlled.
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.
But ... If rebooting the iPad brings the sounds back, you may have another issue going on. Check the settings first and see if that is the problem.
I agree that the only common thread would be the upgrade but the odds of that happening to all three devices has to be pretty high.
Did you by any chance, restore all three devices from the backup of one of the three device? In other words, did you restore both iPads and the iPhone from the same backup?
I just can't imagine that the upgrade would have caused exactly the same issue on three separate devices .... Unless you used the same .ipsw file to update all three devices and the software was corrupt.
How did you upgrade each device? Did you install the upgrades manually?