It is natural for people to talk faster (and sometimes in a higher voice) when they are nervous. The psychology behind it is that speaking fast lends importance and validation to what a person says, at least thats what the speaker thinks. In the other person's mind, it says that the speaker feels nervous talking and feels inferior to the listener or not confident in the subject matter. If you are like most people and want to appear confident in what you are saying, you might need to slow it down a bit.
The next time you are talking, try to notice your voice. Listen to how it changes in certain situations and critique it a bit. When does it speed up and slow down? When does it get higher or lower? Are there certain sounds that come out smooth or raspy?
Then, find a private place and speak the way you would like to sound. It may seem silly at first but stick with it. You will soon find a pace that not only sounds confident but makes you feel confident. If it helps, imagine a celebrity that you think has a great way of speaking and imitate it, then slowly blend it into your natural voice. (For me it was Jack Nicholson, I removed the long drawn out vowels but concentrated on his depth and speed, which is quite slow). Here is an example of Mr. Nicholson Talking. He shows nothing but confidence no matter what he says, as he appears to be doing it on his own time.
Once you have found a pace of speaking that makes you feel confident, go out and use it. Start with people who don't know you. The bank teller, the taxi driver, the grocery clerk or anyone else you bump into. Get used to using it. See how you feel and how others respond when you use it. After some practice, start bringing it into your life until it is the norm. You will be amazed at the results.
DO NOT come up with a fake voice for yourself. You need to be you in all situations to have true confidence. But do learn how to control your pace of speaking and the tone that you use. Having that control will lead to clearer and more direct speech. That will cause you to feel more confident in what you are saying and that confidence will be communicated to the listener.