Want to address a few more of Chris P.'s comments. "Mushin" is an emptiness of mind/spirit as you define, but you stress it as an emotional (or lack thereof) state. This is just a part of a broader, deeper concept. Mushin has more to do with perception, the emptying providing a still surface without ripples, where things can be clearly reflected and seen undistorted (mizu no kokoro). Emotions are just one source of ripples. The main advantage to this clarity of mushin is that it allows "action and reaction to be one and the same." I am speaking here in regards to karate - Mushin may be nuanced differently in meditation or other "do" such as ikebana or kyudo (arts I'm not well versed in.) The kiai serves to startle the opponent, fortify the body and release one's spirit and power during an attack. It must be timed correctly, explosive and forceful to accomplish these goals. Those five second, drawn out kiai as seen in kata competition are not true kiai and serve only theatrical purposes, or to impress uneducated judges. The same goes for kumite when used to tell the judges "Hey, I just scored a point." Regarding the sound itself being the kiai, or just a manifestation of the internal kiai - Can the kiai be silent? A good question. Here is my opinion on it: I think not, sort of. While a sudden expulsion of air is needed as spirit and breath are connected, and in some cases can be silent, in terms of karate the sound is important as a tactical component of the attack to unbalance the opponent. And I also think it serves to help bring out the other elements of kiai as well.