Kiai is a form of indimidation, and in karate you say the name of the kata you're about to perform, or the name of the punch/kick/whatever you're about to throw by shouting it out loud. Also, karatekas (and I would think martial artists in general) are usually discouraged from 'showing off'. Revealing one's skills to the opponent is one of the stupidest thing one can do (also, the opponent might be better than you in hand-to-hand and you'd feel like an idiot for bragging). So, in order, you have to: 1) run away, 2) try to indimitate/scare the opponent, 3) if all else fails attack first as attack is the best defense and make so that your opponent goes down with the first hit and doesn't get up again.
Nunchaku was probably a sort of flail used to separate the grain from the husk (in rice and soy beans) or maybe it even comes from the horse bit. It all depends on whether historicians agree on where was it that it became a weapon first, China or Okinawa.