When I teach voice, I never talk about volume. I talk about breath support. When I tell a student to make something louder or softer, it teaches them to push or hold back without breath or body connection, so it’s all in their throats. What I want them to think about is directing the breath energy around the sound/voicing. When I teach Acting, I talk about making something more urgent or important, and that will naturally increase or decrease the volume. The damage done to a voice comes directly from sound that is not supported by breath. The breath should be doing all the work, and the voice should be doing the articulating. An example might be if I grip the heavy bar for 100-pound weight with my just fingers, I am going to damage them. My hands supported by my biceps and triceps should be doing the heavy lifting. Another example might be, the breath is getting the sound out there, while the voice is doing the communicating. The increase or decrease of volume should come as a byproduct of more or less breath energy and the urgency of the message.
Breath and voice connection comes directly from a consistent practice, which involves warming up and exercising the voice. This work should involve the whole body: First finding where tension is living within the body, then releasing that tension through breathing, stretching, and the creation of sound. Tension is an enemy to the voice, so this work is doubly important to novice speakers because of nerves and inexperience.
Good delivery is meant to augment your speech and help convey your information to the audience. Anything that potentially distracts your audience means that fewer people will be informed, persuaded, or entertained by what you have said. Practicing your speech in an environment that closely resembles the actual situation that you will be speaking in will better prepare you for what to do and how to deliver your speech when it really counts.