Maybe when she does it next time, say something like, "Hah! You must have grown up in the military--you sound so forceful and loud. Effecient with recruits, maybe, but I wonder if my son might listen to you more carefully if you spoke in the softer tones that I and his teacher use. There's really no need to speak sharply to him, and he and you will probably get along better if he's not frightened of you."
but that's a mouthful! whew! Good luck! Don't just let her get away with it, though. He's your kid and she should respect your wishes. There's no need to be harsh to a child if he's not dangling himself or another child out of the window or playing with a lighter on the bus.