Truth in Television: Some people are just always loud, and have to constantly be reminded to quiet down. Let's face it, you have probably met/known at least a handful of people like this in your life.
Little kids. In Pre-school through elementary school, kids are usually taught to use their indoor voices.
It's very common for people with autism to speak MUCH louder than necessary. Because of their lack of social savviness, they may not realize they are unintentionally shouting until this is pointed out to them.
Drill Sergeants in general.
Many DIs are actually fairly quiet when they're not "on", but it's pretty much a prerequisite for the job that you have the capability to embody this trope.
They generally get quieter as their trainees get closer to graduation and become a more cohesive group; but if you screw up, they will be more than happy to remind you that they still know how.
This trope has justification on a parade ground. You have to be heard to relay marching orders over the sound of a brass band and footsteps from boots.
Hans Bauge, a popular Norwegian TV-debater in the 1990s.
Lil Jon's persona in his music is this, with it being incredibly rare to have any lyrics NOT DELIVERED AT A SHOUT. It can thus be quite a shock when he's offstage and proves to be pretty normal, if not soft spoken.
Many singers have to project their voice, as do stage actors. Often this can result in some people talking louder when not on stage. In choirs, since altos are low, they often need to project their voices in order to be heard, especially if there are some loud sopranos who are already projecting their voices at the same time.
This is pretty much a requirement for heavy metal singers, especially in the more underground genres, and forgood reason.note On the other hand, a number of them are quite soft spoken.