Majority of the message that you send to the audience comes from your body language, however your voice is the first runner up. Most Americans get the majority of their information from simply listening, so it is important that the message is delivered properly. This involves several components of your voice.
If you are any distance away from people, it is important that the back section of your audience be able to hear you. Your volume needs to come from your core, not just your voice. That is made easier with good posture and when you are facing the audience. You will want to make sure that you are talking to the back of the room first and look at the people that you are talking to connect them. Nervousness is one of the top contributors to a weak voice; you don’t need the audience’s approval to be a good speaker, so have the confidence to do a great job. Remember, the audience wants you to succeed, they didn’t come to see you fail, they came to learn from you because you are the expert.