Casey
Well, I'm kind of a smart ass by nature. I would do one of two things. I would either go, what? And just keep saying it. I would intentionally be as irritating as possible. Or, I would say, Aww. They're there. Shut up.
Chase Shepard
I would have to say, can you talk louder so the whole world can hear? 
The misfit angel
I just speak really loud and slow back. Maybe even louder and slower just to commit to the bit and show them how profoundly dumb they are.
Emerald
To talk very quietly
Georgian
Sign language, that was a good one Eddie I’ll have to remember that I guess my contribution would be to stick one hand up to my ear and cup it like I can’t quite hear you  and for a little added flourish, you could say come again.