If you're talking too fast, just slow down a little. If you're mumbling, practice speaking to yourself in front of a mirror. If you find yourself always getting nervous when talking to people, just be yourself, may it be weird- smart- or stupid, eventually you'll find the right people for you. If you're not talking loud enough, go someone far, far away from civilization, and just SCREAM A WHOLE CONVERSATION TO YOURSELF. It will make you, and your voice louder. Some side effects include losing voice, and higher confidence.