I have had a few managers in my working career who were exceptionally bad speakers. One would address a group of forty software developers each Friday at a company-sponsored lunch, held in a large meeting area. Some of the developers would be sitting fifty feet away from him as he spoke, and there was the nearly constant commotion of eating, getting more pizza, shuffling, quiet talking, and so on.
But even with these conditions, this development manager never spoke with a tone of voice any higher than he used sitting across the desk in his office. As a result, all in attendance had to strain to hear him, and often missed crucial information -- not only because they couldn't hear him, but also because they had stopped listening.
There is much to be said for a clear, confident voice. Not only is it more practical for addressing a large group, it sends a clear signal that the speaker is the leader, has something important to say, and wants very much for everyone to hear it.
At the other end of the spectrum, though, is something we used to call "loudership". In the hierarchical, bullying world of the military, leaders often resort to shouting. The reasons are the same -- you need everyone to hear you, and you need to assert your authority -- but the tool is not one that you could employ in the typical office environment.
What, then, is the optimal volume for a leader's address? In studying poetry, I found that there is a difference between simple volume and emphasis. When you emphasize a syllable in a word, you don't say it louder; instead, you force more air through your windpipe. I think that this is the key to speaking to a group with confidence; you don't have to shout, you simply need to talk forcefully.
The one thing that everyone agrees on with leaders is that they act like leaders. Although that is rather too tautological to be practical, it is worth remembering that leaders also talk like leaders -- not only to be able to talk like a leader, but also to recognize leaders when you see them.