Just my two cents ramble...
7 years ago when I was doing the Hombu style, I strived to do silent type ukemi, because it was at that time, maybe due to the influence of then sensei, fashionable to emphasize finesse and grace. It was very nice for embu / demo because up from the spectator gallery it looked like ballet, very nice, very graceful. There was also minimal kiai, because it was considered a ruffian to be shouting all around especially in a public area (University sports centre).
Now, after being immersed in the Yoshinkan (the hard style) style for numerous years, my falls are not at all silent. My kiai is always present. The argument put forward by my current sensei with regards to slapping hard on the mat...
Slapping hard on the mat actually conditions ones hand to take hits and blocks. In actual fight, he explained, aikido technique 10%, the initial atemi and/or block should be 90%. What he meant, was the initial hit should in effect be the end of the confrontation (to persuade the adversary to discontinue his/her aggression). The remaining aikido technique is use as an osae (pin / immobilization) to permanently convince that further resistance is unnecessary on the uke's part.
With regards to kiai...from my own observation:
I kiai a lot these days, more so during hard randori. I find that it gives me courage to attack vigorously. It is more true when my shite is some 6 footer and 200 pounder black belter. Second benefit of kiai, it makes me less tired and helps me to stay in the randori game longer. I am actually quite psyche up after a round of good randori. I wonder if the kiai has anything to do with it.
Oh BTW for trivial sake, my kiai sounds like "hait" when attacking and "huh or oozt" when taking ukemi. I don't know why i have preference for such sound. As for some people using sound like "yip" or "yep ", I find it distasteful. To my ears it sound like the sound a puppy make when being punished by its owner.
P/S Incidently during work, without any thought I will also kiai "Lai" whenever a co-worker or customer call me. "Lai" is a colloquial term meaning "here I am at your service"
. Go figure