Mojo
Mostly harmless
Most likely explanation that makes the fewest assumptions is that it's pareidolia. Every single person in the world experiences it. It can be visual or audible; this has been explained time and time again. You cannot just hand-flap away pareidolia just because other people hear meaning in random noise. That is what pareidolia is.
I have said before it doesn't matter how many people hear these voices, that is a red herring.
It wouldn't matter if everyone heard the voices; pareidolia would still be the most likely explanation. Even if they all heard the same messages.