AKA - the bane of an author's existence.
You can plot a storyline. You can research location and vocation. You can come up with a great name for you main character and you can have a solid concept, but without "voice" the book falls flat. Most people mentally "hear" the words of a book as they read it. If you don't believe that, read a best seller then watch the movie made out of it and see if something feels "off" with the casting because it doesn't match what's in your head. Better yet, go to the library and check out an audio book version of something you've already read and see how long it takes you to cringe because the reader's voice sounds "wrong".
Voice is key, but no one really knows how to explain what it is or how to achieve the effect if it doesn't come naturally.
People who read my writing in progress tell me the voice is great, then follow that with an immediate "How do you do that?" I could very easily give them a toss-away (and egotistical) response about how it just "happens", but that's not entirely true.
The truth is, I borrow my voice from others.
People tend to think of writing as a visual medium, and to a certain extent it is, but it's also the current incarnation of the traditional storyteller -- a very audible medium. The bonfires and flash powder may not be part of the average reader's experience when he/she curls up with a novel to step out of life for a few hours, but the foundation is still there. If you're going to be a good storyteller (and you most definitely can be one without being a good writer), you have to be a good listener.
Cadence and rhythm are vital to good writing - this is why one of the most common pieces of advice for writers is to read their work out loud to see if it sounds like real speech. Readers will stumble over "do not" if they know it should be "don't"; it's a visual stutter of sorts.
What I do, and I swear it's as easy as it sounds, is pick a character from a movie or a TV-show and "cast" them as the narrator of a given passage. After it's written, then I read it back in the imagined speaking voice of that character and alter it to fit their speech pattern. For example, the short story a couple of posts down about the life of a wish. In my mind, that's written to sound like the voice over from The Lord of the Rings film (Galadriel's voice). Read it with that in mind and you can make up your own mind whether or not I succeeded.
Hopefully this makes some kind of sense, and hopefully it's of some use to someone out there.