Great speakers know how to find the “low middle” of their vocal range. Most of us speak in our high middle when we’re talking in a business meeting, say, and we rarely change octaves when we speak in public. If anything, our nervousness can cause us to speak at an even higher octave.
Here’s a trick to find your low middle: While making an audible “m” sound at the lowest point in your vocal range, travel all the way up to your highest point and then go back down. Think of a vocal roller coaster. Do this several times until you can hear your mid-point. Then drop your voice two levels below that and speak. Chances are you sound more confident and commanding at that point in your range and you are also naturally louder at that point as well.
Keep practicing until it becomes natural.