Many years ago when I was in high school the choral conductor told me I was singing flat. Since I had sung in chorus throughout grade school, and in public at church, and no one had ever said that to me, and since I had a good ear, I was surprised. Turns out he only like really bright, really forward “placement” and felt that anything which was not that was under pitch. When he left, his replacement told me I was singing in my nose. He sent me to a singing teacher who “got rid of the nasality”. Again, no one ever told me that I was nasal, and that was the summer I sang the lead in Music Man, “Marian Paroo” in a local semi-professional production run by folks from Broadway. While I was still quite young I was told by another singing teacher (of some renown) that I was a mezzo with high notes. [!] Another teacher, at about the same time told me that my voice was small and piercing. All this while I was continuing to perform at local venues to very nice feedback.
Years later yet another teacher told me that the muscles in my throat were tight and that I had a lot of constriction. Since I felt fine while I sang and I was told I sounded “good” both of these statements confused me.
Most of these statements were not helpful to me. They did not really tell me about “my voice”. The one that was most useful was the last one, as that was a functional evaluation: my throat muscles were tight and the sound was consequently slightly constricted. That, fortunately, allowed me to work on something specific that I could improve. It was a relief to know I might improve by doing vocal exercises.
It was typical years ago to evaluate “the voice” as if it were one thing and it could not be changed in any significant manner through training. Whatever one could manage to do while singing might improve through certain exercises but within a rather limited scope. Perhaps it would be possible to learn to go a few notes higher or lower, a bit louder or softer, to have more ability to open the mouth and use “low breathing” along with “forward placement” and “open space in the back” [!] and the rest was about being musical and expressive.
The truth is, of course, that you cannot change your basic anatomy as once you are an adult your physical structures are largely done growing. What you do with your voice can be altered quite significantly, though, if that is your intent and you take the time to allow the appropriate adjustments to have a lasting effect on your vocal output. A light voice can develop more depth and power, a full voice can learn to lighten up and move easily, a low voice can acquire high notes and a high voice can develop a rich lower range. Everyone can learn to sing longer phrases at louder and softer volumes and to pronounce consonants cleanly. A determined vocalist can learn to sing in a wide variety of tonal colors and qualities and in various styles, all while being honest and singing with freedom.
As long as a teacher understands the many complex muscular responses that are possible as we sing, she can comprehend how those movements impact the sound. Since the structures are both interior and exterior and since everything is influenced by everything else, initiating movement in a group of muscles or even one muscle is only possible indirectly. Using exercises as stimulus to change vocal responses, new behaviors can be coaxed from the throat over time. Many things about a voice and how it is being used can and do change over time. Some of them are natural and take place without effort, some of them can be very deliberate and are caused through vocal training. Some things remain mostly the same but other things can really change significantly.