The following comes from "Practice Perfect" by Doug Lemov, Erica Woolway, and Katie Yezzi. Additional comments by me will be in red.
Strong voice teachers use five principles to signal their authority:
Economy of Langauge
Fewer words are usually stronger than more. Being chatty signals nervousness and indecision while choosing words carefully shows preparedness and clarity of pupose. The best teachers I've known have been able to sum up correction in a just a few words. They did not complicate the solution with a lot of words. Just like shooting the basketball -- economy of motion. The less motion on your shot the more opportunity you have to be consistent and accurate -- the same holds true with your words.
Do Not Talk Over
Make a habit of showing that your words matter by waiting until there is no other talking before you begin. By ensuring that your voice doesn't compete for attention, you domeonstrate that the diecsion to listen isn't situational. I would add that talking over means that you are also not listening -- the most important part of communication.
Do Not Engage
Once you have set the topic of conversation, avoid engaging in other topics until you have satisfactorily resolved the topic you initiated. This is actaully an extension of Economy of Langauge. Less is more and helps to keep the communication centered and focused.
Stand Up/Stand Still
In every comment you make,you speak nonverbally as well as with words. Your body can show that you expect people to follow your request. When you want to express the seriousness of your directions, turn with two feet and two shoulders face the object of your words directionly. Make sure your eye contact is direct. Stand up and straight or lean in close (this shows your level of control by deomonstrating that you are not she or afraid; you don't crouch down to a dog you fear will bite you). If the student to whom you are speaking is distant, move towards him. I always think of LSU Coach Dale Brown who take a knee anytime he spoke to a young camper -- I mean everytime. He wanted the conversation to be eye to eye, never wanting to appear that he was looking down to who he was talking to.
When you get nervous, when you are worried that studetns might not follow your directions, when you sense that your control may be slipping away, your first instinct is often to talk louder and faster. When you get loud and talk fast, you show that you are nervous, scared, out of control. Though it runs against all your instincts, get slower and quieter when you want control. Drop you voice. Make students strain to listen. Exude poise and calm. Just like a good ball handler masters that change of dribble, a good communicator knows how to master the change of volume. Sometimes whisper speaks louder than yelling.