You know, that's weird. In the Russian movie about Sherlock Holmes, Watson explains to Holmes that snakes can't hear, and Holmes offers some other solution on the spot. I was sure it was part of the original story. :) Must re-read it now.

But while I was reading Harry Potter, I totally forgot about the fact that snakes can't hear. I think it's because we are dealing with a fairy tale, after all. The idea of a snake language is so cool that one can forgive biological deviations. So what if this universe has hearing snakes? After all, any physicist will tell you that if you are invisible, you are blind by definition, and yet we don't say "The Invisibility Cloak is impossible".