Elmore Leonard used to say that writers just starting out should try to imitate their favorite writers and eventually their own voice would shine through.
...Do you have to work to make a voice your own or should it come naturally?
Anyone entering any field begins by mimicking those with more experience. If you never choose to move past this stage, you will never develop ingenuity and never reach your full potential.
That said, specific to writing, it seems to me developing a trademark voice is only useful when writing recurring characters. If you're writing different characters in different genres, there's no reason they should sound the same. They should instead take on their own personalities, and any overall narrative tone should hit the particular notes of the genre you're writing.
...[T]here's a part of writing that's unique to the writer because every person's perception of the world is different. If you give the same writing assignment to a hundred people, each person will have a different take on it. I don't know that this adds up to voice.
I think every writer has a voice that comes through no matter who or what they're writing about. Not every character has to sound the same or act the same way, but I think a writerly voice often can be found when you read a writer's work... so you can tell it's Author so and so when you read.
to which I responded:
You would only detect the voice you're referring to if you read a lot of the author's work. If you have some idea of the author's tics, you know what to look for. And again, one of the reasons you'd spot these tics is that the author has written recurring characters whose voices are supposed to be consistent.
Does an author's perception of the world weigh enough that it would be detected in everything he writes, specifically fiction? Not necessarily.
Genre fiction isn't a good place to test because, on some level, a writer may be expected to bring some of his trademarks to a new project—to help sell the project.
I don't enjoy detecting one voice behind an author's entire body of work. In fiction, it dispels the illusion of a character's voice; in general, it counters the author's attempts at something new.