In Karate terms a "Ki-ai" is a spirited shout, its use is to enable the person to deliver a violent and powerful movement and at the same time help empty the mind of extraneous thoughts and emotions, It is said that correct use of a "Ki-ai" enables one to liberate mental and physical force rapidly and thus influence an aggressor whom is in close proximity.
Most Japanese Martial Arts use a Ki-ai of some sort and the actual tone or sound will often depend upon the aim of its intention, the actual word 'KIAI' is never used to deliver the "Ki-ai" for the words pronunciation emphasises on closing the throat to utter which would lead to eventually losing ones voice.
Some schools use the word "Ki-ai" simply to remind young students when to actually use the "Ki-ai" but this is soon replaced once the student grasps the importance of its use.
Sounds tend to be more connected to either the intention to sedate or stimulate the opponent into making the wrong move or simply to slow or pause the attackers intentions,
in much the same way that we are all aware of how sounds can directly effect our emotional state in a positiv