Why would the umpire be involved at all? I don't know the details of how these systems are configured but if I were in charge there would be two different auditory signals coming from speakers installed behind home plate, perhaps a beep for a strike and a buzz for a ball or something like that. The other thing about this is that the home plate umpire would no longer need to be positioned immediately behind the catcher, probably about 5-10 feet back instead. He/she would need to be close enough to call catcher's interference, to call whether the catcher held the ball on a third strike, and to easily get in position to make a call if there's a runner on third. Corner umps would make all calls on check swings.