I've just approved my typeset manuscript and re-reading the book, I'm aware that it sounds very far away from my internal voice. Whilst it has all of the elements in practice, the colour and fluency feel strained. Is there a practice or hack that anyone uses to help bring the final product closer to the original dream?
It should sound very far away from your internal voice. Your internal voice is the voice you use to talk to yourself and it has all kinds of advantages that your public voice cannot share, since you know yourself better, and have a complete stock of shared experiences with yourself, that no member of the public can ever share with you.
The art of learning to write is not the development of your internal voice, but the development of your public voice. Our first poor stumbling efforts to express ourselves are simply our internal voice coming out. Listen to a three year old talk, and you will realize that they lack any sense that the experience of the person they are talking to is any different from their own. One of the greatest breakthrough in the development of our language ability is when we recognize that grandpa was not there when I saw the ducks at the park and so I have to tell him about it so he will know what it was like.
Your development as a writer is a matter of refining your public voice, the voice in which you can communicate effectively to people who were not there, who did not see what you saw, did not feel what you felt. This must necessarily be very far from your internal voice, the voice in which you remind yourself what it was like and what you felt and what you imagined.
When we say that a writer has found their voice, therefore, we do not mean (or at any rate should not mean, if we understand the process) that they have found their inner voice, but that they have found their public voice, the voice in which the public can hear them loud and clear.
I don't know if you have made the full journey from private voice to public voice yet, but you should understand that the development of an effective public voice is essential to your success as a writer. The last thing you should do is to try to return to you inner voice. Rather, strive to perfect an effective public voice.
Before jumping to a conclusion about your work, certain things which are not mentioned here need to be considered. Sometimes the writing process can take you out of or beyond what you think of as your "self", and that might not be a bad thing. Give it time to settle and read it again. Such things like the use of a persona, or point of view may affect this.
Having said that, it may well be that you have a sense of the style and effect you want and found the manuscript didn't live up to that. In that case, I would recommend re-reading the parts you liked least (surely some came off better to you than others) and seeing if you can revise them or even excise them. Also look at the work overall and see what it is that bothers you. This is something only you can really assess.
Finally, show it to someone else and see what they think of it. Hopefully that someone will be willing to tell you if something about it strikes them as not quite right or weak in any way. An objective reader (meaning, not you) will notice things you didn't and I've found that extremely valuable.
From my experience (which may not be universal), finding a "voice" requires much writing, maybe over years. I do not think finding the "you" of the soul or life-force is the same as finding your writing voice. The latter can only be done by writing, although finding that voice could possibly help the former. Only by observing your work over time can you get that sense. However, writing something you like is different. I've loved things I wrote before I actually discovered my writing "voice." Or at least something recognizable about it. But it's always a work in progress.
Remember, you might be doing better than you think you are. Or be a good self-critic. Asking the question itself I view as a good sign.
I have often written in a voice that is not my authentic voice. At work and in academia there's a place for that. A manuscript you are not comfortable with is a sure sign that something is amiss. If you think it's your voice, you can try rewriting a bit with the filter turned off, the way you would in journal or something else intended for no one's eyes but your own. You can then compare the two and see which direction you want to take or if there's a hybrid approach that will work for you.
Personally, I tend to write much of my prose in theater form first. In that way, I can put the characters on paper exactly as I hear them in my head. Then I add the prose structure and start editing. In the end it will be quite a different product, but it is the closest way I've found to transcribe my inner voice to paper.