If the owners have them as indoor cats, you should be keeping them indoors. That's really their decision. Once a cat goes outside, the chances of them dashing out gets greater. They aren't used to fending for themselves and are unaware of dangers. Not to mention they may not be vaccinated against outdoor germs and may not have the immune system to handle the outdoors.
That being said, my friend owns a Bengel and he is quite loud! Some cats just are. I had a Siamese a long time ago. Talked so much that when I was on the phone once, the person on the other end said "Go take care of your baby. I'll call you again later!" LOL All you can do is keep them distracted with alot of play.
Claudia