Aiki is the spirit of harmony. Kiai is harmonizing Ki.
You don't need to shout or make noise to reach Kiai. People really mix up Kiai for shouting or making noise, which is a very shallow way to look at it.
Kiai is, this is also a shallow way to put it, calling in all your Ki that is broken apart in order for it to be one. Harmonizing it so it could be a source for your strength.
I use kiai in ukemi not by shouting or make noise, but concentrate on one point. To concentrate on that one point (seika no itten or seika tanden), to concentrate on the hara. To do this, I breathe out, and feel the flow to the hara. Sometimes the side effect of this, for me, is a low gutteral sound.
From a physiological point of view, have you ever tried falling down with a lungful of air? It is very dangerous. Breathing out is quite adequate, but I use kiai so I still have energy after the breakfall.
Just take it in consideration.