Should women alter their voice before giving a speech? Should they lower the tone of their voice?
Now, you can find clinical research that purports to show that women are more credible if they lower their voice. I say that's a bunch of baloney. Here's the real problem. Once you try to start talking in a lower voice and sound like Walter Cronkite, you sound fake. You sound phony. Unless you are a professionally trained actor, it's really hard to change your voice for ten seconds, much less 20 minutes of a speech. This is a huge, huge blunder people make, men and women. Some men are self conscious about their voice sounding too high. An absolute waste of time.
Here's what you want in your voice. You want a full range. You want highs. You want lows. You want louder. You want softer. That's what will make you engaging. That's what will make you interesting.
If you're looking for credibility, why not say something interesting and memorable and useful to your audience. Don't try to sound like this. Don't get your credibility through tone. Get your credibility by actually helping people with your ideas.
Let your voice use its full range, then you'll be in great shape.