Voice is the difference between fiction and a sort of journalism of events that never happened. Strong voice is "Now is the winter of our discontent made glorious summer by this our sun of York," instead of "My brother's victory made me feel good." Strong voice is "It was the best of times, it was the worst of times," instead of "things were mixed."
The single most important element of fiction is storytelling. And that can be broken into having a good story to tell and telling it in a compelling way, i.e. strong voice. It is one of the more difficult aspects of craft to master, and the vast majority of writers begin by copying someone else's voice. To have a strong consistent voice that is distinctly yours is a significant achievement.
There are series of steps where it comes to voice which most professional writers must pass through on their way to mastery:
1. Recognizing and understanding the idea of voice.
2. Writing with any voice at all (usually imitated).
3. Finding a voice of one's own.
4. Using that voice.
5. Doing so with consistancy.
There is a 6th step as well, but it's essentially optional. It is creating voices that are distinctive and personal and that also suit the tone of the written piece perfectly, so that each story is both completely yours and completely its own. That last one is very difficult, and I don't know anyone who does it with real consistency. But 6 isn't necessary to a long and fruitful career or to excellent writing. There are any number of writers whose work I love and respect who only ever go as far as step 5. Whether they could master 6 if they wanted to is, of course, an open question since it has to be exhibited to be judged.
So, any thoughts on voice? Have you found yours? If so how? The steps above? A completely different process? If there anyone whose voice you particularly admire?