Can the defensive players REALLY hear the PA announcers doing play by play during the play?
If they are hearing him, then they aren't doing their job.
Now, announcing personnel BETWEEN plays is a problem. Our home announcer has a habit of trying to mention every kid he can, to make the parents happy (we're a private school, and he's always working on those donations to the Annual Fund!).
We had a punt fake where we brought one of our star athletes back onto the field to throw a pass (Gordon Beckham, now the 2B of the Chicago White Sox). Beckham wasn't normally on that unit, and being the great athlete he was, opponents knew his name. We go to run the fake, running Beckham out there in a group of players. We weren't "hiding" him, but we made sure he was part of a group, and not entering alone.
Our announcer blurts out, "And there is Gordon Beckham going out with the punt team, why that's certainly unusual!" If looks could kill, our head coach would have been guilty of murder that night.