Even an analog switch on the back would be simple and easy to add. I know a lot of them do have ways to turn sound off. But many don't. And a simple switch on the backside would make things so much easier.

There is very little reason that with the digital behemoths that microwaves are now, that a simple “sound off” setting can’t be done.

Most microwaves already have this feature, it's just not obvious how to use it. Read your microwave's user manual.