The correct pronunciation and voice projection of American English depends on an open mouth position. The English speech sounds require the sound resonance created by an open mouth. Speech sounds are made by the changes in mouth opening, as well as lip, teeth, tongue, and soft palate movement, and vocal cord vibration.
Let’s consider the /P/ and /B/ sounds. These sounds are made by slightly rolling our lips under, pressing them together, air pressure building up, and then the lips popping open to release a burst of air. To produce the /B/ sound, we also vibrate our vocal folds. Now let’s consider the /E/ sound as in “beat.” The /E/ sound is made by pulling lips back and open (teeth are showing), the tongue is down in the front and up in the back, vocal cords are vibrating, and finally, there is a steady stream of air.
Each native language has different patterns of mouth opening and movement. To perfect your target language pronunciation, you must use the same mouth position and muscle tension of that language. So if English is the desired language, you likely will need to open your mouth wider. This open mouth will also help you to project your voice better so people can hear your message.
Each one of my non-native English-speaking clients has had to learn how to project their voice better. Our clients have accomplished this by opening their mouth wider, speaking on a bigger breath of air, and realizing they were not speaking loud enough for their audience. These relatively slight changes contribute to their reports of no longer having to repeat themselves.