I was hoping to get a little more information from you, but also didn't want you to wait too long for an answer.
If your son is a toddler, around 3 years old, here are some tips to help him learn how to use his inside voice:
Discuss different sounds heard in your neighborhood, then act them out together. The more your son can understand the meanings of 'loud' and 'quiet', the better he'll be able to repeat them, himself and you can stress that the non-loud sounds are the ones to be used inside.
Tell him a secret--If you whisper an invitation to sit down to have a snack, your son may quiet down to hear you and then, he may want to whisper his requests for more cookies.
Show him it's alright to use his 'outdoor voice' under the appropriate circumstances. The next time you head to the park, tell him, "I'm turning on my outdoor voice now!" This will demonstrate to him that it's okay to be nice and loud---sometimes and only when it's appropriate.
Read a book about the sounds animals and/or baby animals make and together, you can 'roar' like a lion and tell him that's a 'loud' voice, so we don't use it inside, but now, it's okay because we're reading the book; then when you get to a bird or kitten that says 'tweet tweet' or 'mew mew,' that's a soft sound, just like an inside voice.
Tell him that if he's not sure whether to use his inside or outside voice, in a particular situation, to ask you, quietly (like a secret, whispered in your ear).
You can use different adjectives to describe an 'inside voice,' like, quiet, soft, whisper, etc.
While you don't want to stifle his creativity and expression, it IS important to make sure he understands inside and outside voices, so I'm glad you asked this question. Remember, based on his age (which I don't currently know), it might take a little longer or a short time, for him to grasp the concept.
Every time he uses his outside voice inside, remind him gently, with a verbal correction and physical gesture; for example, you can say, "Robbie, we're inside, so we are using our inside voices, now." As you're saying this, use your hand, in a palm down, flat position, to make a motion, pushing it 'down.'
I hope this helps.
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