Spoken communication is made up of a number of factors. Technically correct formation of speech sounds is obviously one aspect and this can be considered by many to be the most important aspect. However a technically brilliant voice is not necessarily one that is easy to listen to.
We need to balance this with expression and possibly more importantly, enthusiasm. As listeners we would, at least, most of us would prefer to listen to someone speak who sounds passionate about what he is speaking about.
Don’t get me wrong, I would much prefer to listen to someone who sounds enthusiastic with an energetic voice but who is also technically correct.
BUT sometimes, what stops a more quiet, introverted person from speaking out is not the ability to learn how to open their mouth wider for diphthongs or how to put the consonant ‘t’ and ‘d’ at the end of a word or knowing when to use the or thee but the  fear of using their voice to convey expression, energy, enthusiasm  or emotion. This is a much harder skill to teach and much more difficult for clients to gain the confidence needed to use it.