I think you're missing the point. Being able to hear or not isn't relevant to the joke.

By definition, chortling makes a sound, whether there is someone to hear it or not. To specify that the chortling is audible implies that there might exist a kind of chortling that would be inaudible, but English already has a word for that: silence.

Let's take hearing out of it, to avoid confusion. Suppose I describe a cat as "sitting statically"; it's another pleonasm, because the implication that something could sit, but not be at rest, is nonsense, making the adjective redundant. Whether there's anyone to observe the cat, or whether the cat can move, isn't significant.