This is an excellent question and one that I've not seen addressed on our boards that often. Ray has pretty much got it down. It's a narrative style, the unique way in which you utilize just about everything you know about the craft, including vocabulary, diction, and way you string sentences together. I love fantasy writers and admire what I call a "Lovecraftian" bent, the language that was used in the middle ages, and some writers can switch gears and write like Verne, and then craft a short which resembles something Hemmingway might have written--short and stark and "common."
Voice is not so much what the writer says, but HOW he/she says it. Like I say, I think it's an accumulation of everything you've learned and the way you express yourself. I steal from many writing forms. Rollins, is an action adventure writer, and I study the way he moves characters in and out of peril--including fight scenes. I admire the beauty in which Poul Anderson describes atmosphere and the phyical environment. I love King's use of metaphor and simili, although this is a heady wine for a new writer and should be used with great care and caution.
If you're voice is "strained" it means you're writing outside your parameters, and sometimes breaks up the continuity of the style, in toto. This happens to me when I research something and add it to the manuscript--it shows that academic bent and style shift. I have to learn to get more "seamless" when performing this.
I purposely set out to copy the styles of Alan Dean Foster, Poul Anderson, and Peter Benchley, because I fell in love with the way they conveyed thoughts and expressions. Of course I never reached a point where I could ever duplicate them. But what I found was that I am the sum total of all of them, with my own mindset thrown into the soup. Therefore, I AM unique, and think that I have discovered my own voice.
Voice can also be attributed to the way you handle characters, and King does this pretty well. Of course, with age, comes experience and a great sense of irony.
Voice grows over time. It's been said that J.K. Rowling was pretty much voiceless with the first book, (simple writing) but then you could actually follow her through the later editions and watch her grow and mature as a writer, where she stretched her literary legs and took more chances.
There are so many flags that define voice or set it apart, it's hard to nail down any one thing that is specific. But I would say that anyone who uses a lot of sentence fragments in their writing (and gets away with it), has developed (or habitually used) a style that works for them that can be seen in all their stories. This is just one example, but I think you get the picture.
I once started a thread here that was called "Who do people say you write like?" It was fun discovering something about yourself that you weren't aware of. Some of the writers confessed that they were suprised by other's opinions of their style. So voice/style, can be something that your unconcious mind dregs up and lays down over time. For instance, two literary editors tagged me as a cross between King and Roald Dahl. What? I wondered. Does that mean I have long, run-on sentences with lots of bubble gum and candy in there? Other's impressions of your voice will nearly always be different than your own perception. And that's when you'll know that you have arrived and are your own unique creature.
But I can't help but dream that one day I might morph into Poul Anderson. The true beauty of it is, I never will. I'll be me. Then somewhere down the road of time, a teacher might exclaim to her student, "Dang, you write just like Triceratops Stevenson. Well done."
Alas, writers are liars and literary thieves. And it's so great we don't get busted for it.
Tri