No, I don't think a horn would work on free range chickens. They seem to have minds of their own. It's funny how chickens can be scared of something little but something that they are used to doesn't phase them.
I fell in love with chickens last winter when DH tried to back our 35,000 lb truck into a customer's driveway and the neighbor's free range hens would not move out of the way. I would have thought that a large truck would scare them. They weren't afraid. I had to get out and pick them up to move them. They were so sweet. I now have chickens too.