I often find myself unconsciously mimicking the style of the current book I am reading, that is, if I'm not working on a specific project with a specific tone. Sometimes it's a good thing, other times, not so much. It can skew the direction of a story, either derailing it and causing a train wreck or changing it for the better.
Voice is a significant aspect of a story, and finding your own unique voice takes some time and some practice. When you find your rhythm, you know it. Many times, and usually every time, it changes depending on the story you are writing.
Writers almost always say that to be a good writer, you have to read a lot. I read so much it's ridiculous because I want to improve my writing. By seeing what others have done, what has worked and what hasn't, you can shape your writing (and reading) habits, and your own voice.
Some writers that have contributed to my own unique writing voice over the past few years are Martin Amis, David Foster Wallace, Joseph Franzen, and Joseph Conrad. They are obviously super famous, and I will never be as wonderful writers as they are, but I like to see my writing style in the same vein as theirs (though Conrad is quite different from the other three; I am influenced by him theme-wise [i.e. Heart of Darkness]).
To what extent are you influenced by other writers? What writers have influenced you?