“I think your motor is going too fast, you need to slow down and use your indoor voice”
There are many ways to help children know that they are speaking too loud or two quiet. One of the ways is is to model for them how they should be talking, either by speaking very loudly or speaking so quietly that you are almost inaudible. With older children using video feedback is very effective. Record them, ask them what they think, and 9/10 times they can tell if they are too loud or too quiet. It’s very important when they are using an appropriate volume to let them know by giving them positive feedback. This will help them know when they need to turn the volume up and when it needs to go down a notch or two.