Imagine yourself speaking, your voice loud, clear and confident. Visualize the audience clapping – it will boost your confidence. Close your eyes and imagine the most confident and well-spoken version of yourself up in the audience, dazzling them with his words. Or if you're nervous about speaking in a smaller social setting, picture yourself wowing a small group of friends with your words. Picturing the scenario you want to enact can go a long way in making you succeed.
Speak loudly enough for everyone to hear. While you don't want to shout, you should speak loudly enough so that people don't have to ask you to repeat yourself. Speaking quietly or softly will make people think you are shy and that you don't have confidence in what you're saying -- that you don't really want to be heard anyway.
If you speak in a low voice, not only will others not be able to hear what you say, but you will also portray a submissive demeanor, which suggests the opposite of a confident one.
On the other hand, you don't want to speak so loudly that you're talking over people just to be heard. Your words alone should get people's attention.