Once upon a time, before even shouting "booooo!" seemed to be the universal sign of disapproval (or is that... approval?), hissing was an established and socially accepted way to register ones discontent. Nowadays, of course, it survives principally as a certain bit of spoken slang, but is no longer a singular act of disdain by the masses. Goodness gracious, why not!? As recently as 1917, some folks were jailed for exercising their right to voice unvoiced fricatives. Do we not owe them something, some sense of sincere esteem?!
I disapprove (sssssssssss!). A hiss in a modern socially polite context wouldn't be any more acceptable than a loud chorus of "boo!" (or a communist), but at a raucous event such as a sports match or a presidential debate, a hiss would resonate loudly and strongly; however, its placement high up in the frequencies audible -- well over 10kHz -- keeps it from interfering too much with the proceedings. The human voice resonates at a spot much more comfortably couched at less than half this frequency, so speakers could continue on without being significantly drowned out. Would not this, then, be a more polite way of conducting business? I aspire to a more prim and proper age of group dissent, when it was slightly less disruptive! I also have a wholly undue admiration for top hats.
There's also an interesting speculative connection between hissing in disapproval as a goose would and the phrase "flipping the bird." Certainly is a tantalizing possibility!