This is quite interesting, it seems to go against everything that is taught in Iwama.. surprise lol.
Morihiro Saito used to say that you cnat practice real AIkido or any Budo without Kiai. In this interview from http://aikido-france.net/articles/saito5/
When asked "What is the significance of kiai?" he answerd:
Morihiro Saito Sensei: Kiai is something that comes completely naturally when you have ki or energy. It comes by itself. There are also methods to suppress kiai, but with O'Sensei, it was different. He said that you should shout from the abdomen. Indeed in martial arts a big kiai is a sign of good energy. It is important to make kiai. Training without kiai does not make big progress. It is not powerful and has no energy. With a big kiai, a training is full of energy. That is why it is better to shout out loud. O'Sensei always said, "What kind of kiai is that? Make a bigger kiai! Go outside and let the sparrows fall from the trees!" He always said proudly, "When I once made a kiai at the newspaper company, Asahi Shimbun, fourteen people fell from their chairs." When you have to train where you cannot make noise, for example because the neighbors complain, you can't help but be silent. But wherever possible, you should shout out loud.
His son Hirohiro Sated in his new book that KIAI is the tool that connects/unites the mind, body and spirit. He says it goes into Shinto religion and prayers and Kiai is connecte dto it.
Ive seen people in Iwama kicke dof the Mat because they wont learn how to Kiai. H simple says teh sma as his father that Aikido cannot exist without KIAI.
What about all the Videos of Osensei that have sound? they all have him using Kiai. He was famous in Iwama as the crazy old man with the scary Kiai.
Personally I coul dnever imagine doing Aikido in silence, ive tried it at other clubs and the art feels dead to me.