Does a dog-whistle not make an ultrasound? There are many examples and applications where sound has very legitimate but inaudible effects. Perhaps this definition has contenders? Seems to me there are audible and inaudible sounds. And since definitions tend to follow the masses regardless of logic or sensibility, "sound" probably should also mean "appear", as it's become an idiomatic (non literal) term you'll see peppered throughout Hackernews and elsewhere, e.g. "this sounds good; this sounds reasonable;" etc, used to describe one's interpretation of texts, which sans synesthesia, shouldn't be audible.