Here’s my two cents’ worth!
Authors have ‘voices’ that are as different as musical instruments – one may be a flute, another a bassoon. Even an individual author can have several voices – you write a letter to your lawyer in a different voice from a letter to your lover (at least, I hope you do!). In the same way, your ‘voice’ in a blog like this is different from your voice in a romance novel, and your voice in a romance novel is different from your voice in a crime novel.
Would-be writers sometimes ask how they can ‘find’ their voice, or even how do they know if they’ve found it. Some beginners think they have to ‘sound’ like a writer, so they may think they have to write long sentences with flowery or 'literary' descriptions. In doing so, they’re in danger of losing their own natural ‘voice’.
I’ve also seen advice about studying other writers’ styles in order to ‘find’ your own voice, and I always cringe at that advice. A writer’s voice isn’t something that can be learned (or copied) from others. It's already there within your writing, it’s YOU. The secret is to relax and let it flow.
That’s not to say it can’t be improved, by learning about the technicalities of grammar and sentence construction, of course. It’s also said that the more you write, the stronger your voice becomes. Basically, however, your ‘voice’ is how YOU write. I like to think of it as writing from the heart, from the hidden depths of the inner ‘you’. One piece of advice I like is 'Write from the heart, edit from the head.'
In a website about writing for children (Write4kids), I found this paragraph, which I think applies to all books, not just children’s books:
The elements of a book - the plot, characters, setting, description - are all important, but alone they make up the bare bones. With a voice, a book becomes more than words on a page; it becomes a story. The writer's voice breathes life into a book and gives it a soul.