The reason most kids or children struggle with the volume of their voice is because the child or kid does not have a fully developed brain.
The child or kids brain is still growing and developing which makes it a struggle for some kids to control the volume of their voice.
The child also has to learn about the appropriate level of their voice they should talk in and you have to teach them.
When the child comes yelling or bursting out with some loud voice you need to remind them to lower their voice.
Not all kids do struggle with their voice though and some kids are actually pretty quiet and know how to control the volume of their voice.
But some just need time to learn about when to use a loud voice and when to use a normal voice.