Put simply, a writing voice is what makes Hemingway sound like Hemingway and Stephen King like Stephen King. Everyone in the business of novel writing has one, including you.
Voice is made up of many ingredients...
How do you find your writing voice?
That's easy: you don't! Why? Because you already have it. You just need to learn to use it properly. And you do that by not forcing it.
You know how some people have a normal speaking voice and a "telephone" voice? Well, when you write, never put on a telephone voice.
If you try to sound literary because you think that is what novel writing demands, you won't impress anyone. If you use words you have to look up in the dictionary, your readers will have to look them up, too - and they really won't thank you for it.
You must only use language that comes naturally to you, and write in a natural style. Write like you speak and you won't go far wrong.
Of course, you won't write precisely how you speak...
But the purpose of this polishing is to make the prose sound more natural, never more literary.
Words are simply the tools you use to convey the meaning of a novel. It is the meaning that counts. And the best way to get it across is by using a simple, precise writing voice.
"You say what you want to say when you don't care who's listening. If you're grasping to get your own voice, you're making a strained attempt to talk, so it's a matter of just listening to yourself as you sound when you're talking about something that's intensely important to you."
- Allen Ginsberg
"A good style should show no signs of effort. What is written should seem a happy accident."
- W. Somerset Maugham
"The whole secret of a living style and the difference between it and a dead style, lies in not having too much style - being in fact a little careless, or rather seeming to be, here and there. It brings wonderful life into the writing."
- Thomas Hardy
A look at the importance of writing simply in Avoiding Purple Prose