Heero just gives Duo a long look, because it's pretty apparent that he hasn't gone deaf just by nature of having this conversation. Obviously he can hear, because no one is that good at reading lips. Heero is... very literal.

He can't argue with Duo's observations regarding the Peacecraft family's tendency to throw themselves into increasingly dangerous situations, though. With different intentions, of course, but the end result of almost getting killed on multiple occasions was the same.

Also, that whole collapse/destruction of the kingdom over their commitment to total pacifism. "In the DNA, I guess," Heero comments.