Voice is not only your sonic identity, but is also a social indicator.
The way you speak contributes to those all important first impressions interview & performance coaches talk about so regularly.
There are ways & means by which you can improve both aspects, but change begins with awareness. This awareness is essential in order to be able to make changes…you can only change behaviour of which you are aware.
Voice is a learned habit.
By this, I mean that we learn to speak from those who raise us, parents, guardians, grandparents etc. our acquisition of voicing starts whilst we are still growing in the womb & continues until the very end of our lives.
The process of language acquisition is a slow & multi-faceted process one & we only learn my trial, error & correction. We are all born with the facility to speak any language (after all, there is no difference between the vocal anatomy of human beings across the planet), it is only the society we are born into that determines how we communicate with others. Learning languages foreign to our mother tongue can take a long time, not just because there is a different vocabulary to contend with, but also because the way that sounds are produced in that language may be unfamiliar to your current setup.
This setup can be changed once you are made aware (by someone else or by your own ear) of the difference that exist between pronunciations of words. Even when you understand these differences, it can still be a protracted process.