I've read somewhere that an author doesn't have a voice or true style until they have written over a million words. This is true to a certain extent. By the time we've written that long, we've stopped trying to copy our favorite authors or second guess ourselves, etc., if for no other reason than we're tired of doing that.
Some writers don't read the kind of fiction they write while they are working on a book for fear that they will start copying a writer's voice instead of using their own.
Voice is more than just the use or misuse of metaphors, etc. I know I choose the language I use because of the character's viewpoint I'm in. (I write strict third-person viewpoint.)
One character might see a plane wreck and describe it in my narrative as
*The plane's pieces were scattered over the valley like clothes dropped by a drunk on the way to bed.*
Another character who is more analytical would think
*The gouge of earth left by the plane's moving fuselage led him to a boulder. The left wing tip lay against it. The furrow veered violently left there, and bits of wing then fuselage littered the area around it. When there was nothing left of the plane to break apart, the gouge ended.*
The author must also choose voice by the genre expectations of the readers. Choosing the wrong voice can be quite jarring.
Can you imagine a romance novel written like a "noir" detective novel.
*I can say this for Lord Garven, he was built, built like Cleopatra's Needle, but I walked away alone in the dark, dank London fog. I had my partner to avenge, and he had a date with Lord Southby.*
One big mistake I've seen used by beginning writers is emulating the wrong writers, especially writers from the past.
A friend had this thing for Sinclair Lewis who wrote in the early 20th century, but I had to explain to him that Lewis' style was hopelessly outdated with its languid pace, florid style, and sentence structure, and with the current tastes of editors and readers, he would find no readers.
It's equally disastrous to emulate the current literary style of the moment like writing in first person immediate.
*I look at Lord Garven. He is built. Like Cleopatra's Needle. But I shake my head no and walk through the door. I must find my partner's killer.*
By the time you're publishable, the moment is long gone.
What I'm saying is find the right voice for each work, and your own voice will emerge.
A NOTE ABOUT "VOICE" AND EDITORS AND AGENTS:
These days, some editors and agents will tell you they don't like your voice. What they often mean is that you aren't writing in the current popular voice of the genre.
For example, most urban fantasy with a female protagonist is written with the chatty, sarcastic tone of first-person chick lit. Some really excellent and popular urban fantasy is written in third person, but, unfortunately, most editors seem to ignore this fact.