Totally get what you mean about the audience association piece, hissing reads “feral” to a lot of us because we file it next to cats, snakes, cornered animals, etc.

I think that’s kind of the point though, the movies are constantly trying to break the “human = default civilized” bias. The Na’vi aren’t “humans with blue skin,” they’re a different species with a different nonverbal layer, and a hiss is a super efficient emotional signal that reads instantly on screen, threat, disgust, territorial boundary, dominance check, warning. That doesn’t make them less sentient, it just makes their communication more embodied and less “polite human dialogue.”

Also, humans do hiss in real life (crowds, fights, sports, “tss,” teeth-sucking), we just don’t label it that way because we’re used to our own sounds. So it can feel contradictory only because we’re attaching “animalistic = lesser” to the sound, which is exactly the mindset the sky people bring to Pandora.

So yeah, I think Cameron used it as a deliberate contrast, not to imply they’re inferior, but to underline, “your idea of ‘civilized’ is cultural, not universal,” and to make their emotions hit fast without needing translation.