In music, we deal with dynamics: how loud or quiet your sound is. Dynamics are usually relative, being called soft, medium soft, medium strong, and strong. In Italian, it is piano, mezzopiano, mezzoforte, and forte(p, mp, mf, f). You can also go softer with pianissimo (pp) and stronger with fortissimo (ff). And you can keep adding p's and f's for more dynamics.
While reading music, a dynamic symbol tells you to change your volume. You can go from any volume to any other one depending on the symbol it shows. Much like in speech, it can show off certain sections or accent the importance of others.
Sometimes, we want a gradual increasing or decreasing of sound. For that, we have a symbol called crescendo to get louder and decrescendo to get softer. The symbols just look like < and >, but are stretched out for as long as the change lasts.
When you are speaking to people, you do want to accent or highlight certain parts. But to go from a normal volume to a particularly loud volume suddenly is jarring and unpleasant to listeners. It makes you seem crazy and/or uncontrolled.
If you instead crescendo into forte, the gradual volume increase will prepare listeners and build their interest and excitement for the major point you are going to make.
Public speaking is a form of communication, and usually is done with a written script, but it truly is its own art. Interestingly enough, it does borrow a lot of principles from music, so the more you study one, the more it will help you with the other.