It's meant to be overheard (5,7)
I believe the answer is:
stage whisper
(Other definitions for stage whisper that I've seen before include "Actor's quiet remark meant for the audience to hear" , "Actor's aside" , "Ostensibly private aside intended to be heard by audience" , "something the audience should hear" , "Utterance by an actor, supposedly not heard by the other players" .)