I just went through this with my 3 year old. He knows when and where he can be loud because we always enforced the command when it was important, as in one clear warning such as: This is the library. You talk quietly in the library. This is your only warning." Then, if he started to run and scream in the library (what kid wouldn't at least once?) Off we immediately went to the restroom for a good stinging pop on the butt. Since this is consistent with how we handle situations where he will always need to behave a certain way, church, grocery store, people's homes, our home if someone is napping, etc. it only took once, because he was familiar with the drill since age 18 months or so. Now I can take him anywhere and he's always good at the library. He's really good about being quiet when he's supposed to be, and I'm really loose about letting him run and be loud a lot at other times (more than my husband cares for :) because he's a boy with lots of steam and in winter we don't go outside every day, etc. Sometimes he's allowed to run and scream like a banshee in the house. Sometimes the 3 kids are allowed to yell the whole time I'm driving somewhere. But if I say stop, they stop.
Anyway, I've been trying to do yoga every day when my baby naps with yoga sessions from 30 minutes to 90 minutes long. I know it's a lot to expect a five year old and 3 year old to be quiet that long, (if they wake the baby, the session is over) but it is possible. So I make sure they're well fed and have had some time to yell and be loud already before I begin. I set a side a specific area where they can play and a couple of fun activities there so I wasn't just expecting them to sit still and quietly and stare into space or something. The first class I did, he started "trying to do it with me" but would fall, flop down on purpose, start running around, and eventually start being loud in general. All the while I was shooshing him and telling him to be quiet, not distract me, and not wake the baby blah bah blah. Which I know he understands. 3 sessions/days into it, OF COURSE he was still doing it, because he had learned that mom does not ENFORCE when she is doing yoga, she just shooshes.
I did not want to break my zen mood and pause the DVD and interrupt the peaceful yoga music to sting a butt, but basically, if I wanted to continue to do the yoga, it's the only time of day I can, it's been helping a lot with my energy level in general etc. So in order to "teach" that this was also a "no loudness zone" I gave him one more last clear warning that this was like the library, etc, he was to be quiet and play X for X amount of time and not wake the baby and if I had to tell him one more time he would have a consequence." Of course he didn't believe me (my fault for lots of false threats) so he got a firm swat the next time. Now he's super good during yoga, and we all go do something fun together after.
You have to enforce. I never knew a naturally loud child to be "quiet" because someone told them to be. You have to get very specific about where and when you need him to be quiet, not just constantly tell him to be quiet everywhere. Then use your firmest most effective discipline to enforce after one calm specific warning. This is to deter future episodes. If you do something more lenient, you will continue to have to manage and discipline it all the time. You want him avoiding discipline, not cooperating with it.