Here is a great FREE tip on how to improve the quality of your speaking voice - immediately!
Relaxation is the key to a successful and powerful voice, however, when we are nervous, it is hard to relax.
Good breathing is the key to a good voice as the breath is the initiator of the sound, i.e. the force that strikes the vocal chords forcing them to vibrate. It is important to learn how to use your breath properly rather than just taking lots of deep breaths.
When you are nervous you do not breathe enough, which means you don't think so quickly in fact you can become crippled by your nerves, which in turn affects your voice and stops you in your tracks, which makes you even more nervous.
Some nerves are good, they keep the adrenalin flowing and the mind working quicker. Nervous energy is a different thing from nervous tension - one produces energy and the other wastes it!