[simplyesl]

Here's what i've done... I've found it to work quite well:
I pretend that I can't hear what they are saying. They usually get the message and read louder. One class i taught was a bunch of 11-12 year olds, with one 15 year old. I think he was a little put off by being around so many younger sts. I actually left the classroom and yelled in "Read so that i can hear you!" Eventually he read louder and louder. It worked well.