Oh man, I can relate to this. Add this to the mix, my mother is almost deaf and my father is extremely hard of hearing. They both wear hearing aids and my father mumbles. I spend dinner at their house acting as a U.N. translator. As far as jokes go, they have great senses of humor but by the time I've repeated the joke a half a dozen times I just lack the gusto to give the punch line any enthusiasm at all.

It ends up with me just looking at them and smiling which they then use as a cue to laugh.

On the plus side, I don't have to work too hard for a laugh when I'm with them.