Beer

The more you drink, the louder you get.

According to my responsible adult anyway.

The man Bridge has a point in general, but not really in terms if volume. Professional coaching may be about a style or two that doesn't accord with folk, (at school, I recall my violin teacher getting us to play jigs and reels by stripping the soul out of them to show Vivaldi is superior,) but in terms of volume technique, a good tutor knows how to make yourself heard over an orchestra.

So in volume training alone, don't knock classical training techniques. I may not be the best singer in the world, far from it, but luckily, if I need lessons in anything, it's how to sing more quietly....