I read part of a post by another author about how she struggled to find her voice. She referred to it as "knowing which character to listen to", which strikes me as a pretty good description.
Voice is interesting to define. I guess you can call it your signature way of writing, your MO that sets your story apart from others'. Isaac Asimov, sci-fi writer, wrote a lot of stories that hinged on logic, ran short on description, usually sci-fi stories and usually with a hint of mystery and a neat twist. Diana Wynne Jones writes character-driven fantasy that almost seems to have no particular plot until everything comes together at the end. Terry Pratchett writes hilarious parodic fantasy that frequently throws a philosophical question into the heart of the story.
I've never worried a whole lot about voice. For one thing, I didn't know it existed for quite some time, and ignorance was busy bliss. For another, I was busy figuring out plots and making the main character more interesting than the sidekick, and keeping him alive, and borrowing from whatever author I was currently reading.
Nowadays, I know about voice. I know it does, in fact, matter. I know it impacts plots and main characters and influences and all that jazz. So what would I consider my signature style?
I've been thinking about some of the stories I wrote when I was younger. I had two kinds, mostly: the comedy story that veered into drama, and the drama that veered into comedy. My heroes are all snarky, quick-talking, ironic, and typically have bad things happen to them. Hubris is usually involved. Bad guys tend to be very bad. Love interests are fair, feminine, and pretty decently capable. (You can't have a character who stands around wringing her hands...or his, for that matter. You just can't. Unless they die quickly and amusingly.)
Here's the thing: voice needs to transcend character types and story preferences. One of these days, I will write a story (two stories!) about a quiet guy who's bad at comebacks. A girl who's a tomboy. I'm already working on a quasi-sympathetic villain.
But in the meantime? I can have a snarky, ironic, languid hero, who's terribly posh and can't be bothered to take fencing lessons. I can have a snarky, ironic, active hero, who kind of hates the world and himself and doesn't want to admit he's lonely. I can have a busy, feisty, plain heroine, who will SLAP you for wringing your hands uselessly, and a feisty, somewhat silly, girly heroine who also likes sword-fighting.
And villainous villains. Those guys are still interesting.