Praise them excessively when they talk in an "inside voice" or whatever it is you call it :) If necessary you could start with tangible rewards (like one m&m or something) and then back off to social praise once they start getting the hang of it.

My kids did the same thing for a long time. So, I started whispering whenever I talked to them. They were so intrigued by it, and they had to be quiet to hear me and soon they learned the "inside voice" technique. Sounds silly, but it worked. I still remind them "no screaming unless the house is on fire."

My son has spurts of this as well. I have to sit in the room with him when it's time for him to go to bed and I've gotten to the point that I grab my Ipod and ear buds and put that on so I can ignore his outbursts. He stops pretty quickly when he realizes I'm not listening to him. You might try ear phones and their own music players for them. Logan calms down when I do this for him. It calms both of us down. lol

My son is 14 and still forgets sometimes and talks too loudly. He can also be an incessant talker. I used a hand signal when he was younger (I tapped my thumb and forefinger together) to cue him that he was talking too loudly. Using this helped me not to have to try to talk louder than he was so he could hear me.

Now that he is older I remind him that if he can touch the person (or people) he is talking to he can use a quiet voice. For the constant talking, which is usually a monologue about a special interest, I sometimes have to tune him out and let him go. It is almost as if he has to off load the information from his brain. Other times I turn away from him so he knows I am not interested in what he is saying. Sounds cruel but his classmates can be even more direct (Shut Up). It has been a long process.

may I agree, my daughter just learned a few days ago to imitate low voice"" and she thinks is funny,, but she knows when I say I ll push" your shhh button"" lololol