Keyboard not typing the letter M

Keyboards are cheap and a standby el-cheapo USB one is always a good idea. I suggest you get a cheap KB and resume production while you work on the problem.

But keep in mind, Apple is not going to want to repair your KB, they're going to tell you to buy a new one. KBs are like batteries in hat all are different and none last forever,

FWIW, it sounds like you accidentally assigned the letter m to a shortcut. That would explain the beep. Don't know how you would be able to fix it.

Just to add, trying a different keyboard is fine but if I'm correct, it won't help.