often AS people are unable to realize or regulate volume, pitch, tone, inflection etc.
next time he is innapropriately loud, ask him if he knows he's being loud.
buy your son some earplugs and show him how to use them, and where to keep them. He ought to appreciate them, and the fact that he has a prepared solution set up for the next time he's over-burdened by her screams should help avoid a meltdown altogether.
If it doesn't head for the earplugs, consider it might be a tantrum not a meltdown.
Consider it might also be the earplugs are uncomfortable.