Voice is simply the way you write. It's about your choice of vocabulary, your word order, your writing style, your subject matter. It's as individual as your speaking voice and comes as naturally. You can tweak it, in the same way you can tweak an accent, and you can develop it through reading and writing, but essentially it is what it is. Your voice is you, and there's not a great deal you can do about it.
But sometimes your voice doesn't match your chosen form. When I started I wrote literary short stories. I had some success with them, but the natural home for my voice is contemporary women's fiction and when I started writing in that style - by accident! - it just clicked. That's one of the good things about writing exercises; they force writers to go outside their usual genres. Sometimes that suits the author's voice better.
Writers with very distinctive voices have to be doubly persistent. They can struggle to find a form that both shows off their style AND fits into a publishing category. I'm afraid that publishing is a cautious business and they like work that can be neatly slotted into marketing boxes. Work that sits outside the usual categories struggles to find a home. Kate Atkinson and Terry Pratchett have such distinctive voices that they have developed genres all of their own, and there are many other similar examples.
Sometimes authors find their work being published in areas they didn't expect - last year I met a Costa winning author who hadn't expected to be marketed as YA, she just wrote a book. I've heard other authors who thought they'd written one thing, but were marketed as another eg Louise Welsh and The Cutting Room. Jill Mansell once told me that she'd tried writing for Mills and Boon, and kept on being turned down because she had too much humour. She finally changed to chick lit and rom-coms and became one of the top selling UK authors.
So, don't worry about your voice - it's there regardless - but search for the form that suits it best. It may not be what you think it is.