Most FMA stylists tend not to yell, but I've always found the concept of shouting an odd concept in a fight anyway. In boxing, the quickest way to get knocked out is to leave your mouth open. If you get hit on the jaw at with your mouth wide open, it often causes dislocation and/or knockout. If ever I'm sparring, or look like I might have to defend myself, the first thing I do is to shut my yapper, and get my chin down.
It has always surprised me when I've seen karate practicioners attacking people with their mouths wide open, yelling, and their chins up. This just goes against everything that a boxer is taught. FMA stylists often tend to have similar attitudes about combat as boxers. Techniques are not used unless they are effective, and fighters tend to have a no-nonsense attitude about an engagement. Taking this into account, I'd guess that many FMA stylists would wonder exactly what the purpose of a kiap is, and how it will help them in a stickfight where the exchanges are very rapid multiple strike flurries.
Unlike some other systems, most FMA fighters never count on the one strike dramatic kill that some martial artists seem to think will happen. This seems to be where some systems insert their spirit yell of whatever variety. But the truth is that it's darn difficult to actually drop an opponent with one shot, whether you are yelling or not. The Dogbrothers are perfect examples of this. I remember when they were just getting going, a lot of people thought that they'd end up killing each other with one deadly blow. After all, they were well trained martial artists who were intending to go all out. Well, they pretty much proved that the human body can take a lot of punishment, even when subjected to the "kill shots" that some people thought would finish someone. The fact of the matter is that you can often fight your way through these shots, whether someone is yelling or not. And chances are that once you do fight your way through, the person who was yelling will just be a bit more out of breath.
I tend to make some noise out of habit, usually when I'm throwing combination strikes, but I do so between gritted teeth. It's definitely not dramatic yelling sounds, but usually grunts or little exclamations such as "Bang", for which there's no real reason for doing. It's just more satisfying when I whack someone and have a little sound effect help entertain myself.