When someone does something rude, I like to put the behavior and awkwardness back on the rude person. I would say something like, "You seem to find deafness funny. Why is that?" If there is a real, legitimate reason, the person will tell you and you can clear up any misunderstanding. If there is not a legitimate reason, then whatever reason that the person gives makes her look like a jerk (putting the attention back on the jerk and not the person being bothered by the jerk). You don't have to be aggressive, just ask like you are genuinely curious. Just ask a direct question and patiently wait for a response.
This technique works well with all sorts of inappropriate behavior, like noisy people that ask how much you paid for something, relatives that say snide things, strangers who criticize your child rearing in public, etc.
For noisy questions, remember this simple question: "Why do you ask?" Usually, the rude person stands there with her mouth open not having any idea what to say. That's when I make an move to get away from her.
