Vocal coach Roger Love wrote in his book “Set Your Voice Free”:
The reason we need t develop the voice is to allow it to be as expressive and flexible as possible because when you do that, you’re setting your voice, and yourself, free.
If you have issues with posture, then you likely have issues with being heard as well, as both have phyco-somaic correlations.
According to Namgyal Rinpoche, the voice and posture are two of the most difficult things to change in yourself as well, because they are so habitual. They way you think about yourself has been encoded in you your entire life, and will show up in how you carry yourself and your voice. And then how we carry our selves and our voice feeds messages back to our brains, reinforcing the same messages.
Roger Love also points out how our voice affects what others think of us:
The sounds coming out of your mouth set up a whole range of expectations about how you’ll behave, how accessible you are, what your sense of humor is like, and how high your energy level is – to name just a few of the qualities we encode in our voices.
As noted, how we speak is deeply habitual. When something is a habit, it is also usually unconscious. You are probably also unconscious sometimes of how your voice is affecting others reactions to you, either negatively or positively.
The nice thing is, because of this psycho-somatic relationship with the mind and body, we have an opportunity to change the way we think about ourselves, just by changing the way we speak.
Think the voice isn’t flexible? Check out this video: https://youtu.be/1AUglmXa3b0