A story has a voice and a style, and it suits that story and no other. That's a voice... wonderful tone and style and fitted the character perfectly.
I have to disagree with you on that.
Every story has a storyteller. There is always a narrator, whether that's an omniscient author or a character moving about within the book. The "voice" is how the storyteller uses words to get the story across.
Every character has a different way of looking at things and using language. For some authors and some stories - not by any means all or even most - the aim is to make the author disappear so that the story is told in the language and vernacular and thoughts of your Point of View character. Shall we call this the Holy Grail of third-person close? In such a book, every description, every action, every introspection comes directly from the POV character's head.
But most characters are not storytellers. What if you want to write a novel that tells the story of an 1800s coal-shoveller? That's not an educated man. It's probably not a very thoughtful man. A novel written by such a man would consist entirely of short sentences, simple, describing exactly what's happening and not much more. How intriguing can such a book be?
Take the same story and tell it using the (educated, literate, thoughtful) author's POV. Sure, when describing things the POV character sees and thinks, the author uses language and thought patterns appropriate to him. But the story is a springboard from which the author can describe the milieu that surrounds this man and the plot.
Even the most immersive third-person narrative blends the character voice with the author voice. Your coal-shoveller novel will doubtlessly use sentences, possibly even words and metaphors, that would not normally come to mind for this uneducated man. Unless you're Raymond Chandler, you're not likely to completely sit your story inside a character's head - and even Raymond Chandler had an identifiable voice.
If your story contains more than one POV character, you don't want a novel that swings wildly between different forms of language. Readers will get whiplash every time they start a new chapter. You want a novel that reads like it's written by one person, the author. Or if you're not writing a third-person close or a first-person narrative, the author has a voice. What voice is "appropriate to the story" then?
Voice is one of the things readers look for when they pick up a book from one of their favourite authors. People don't pick up a Stephen King novel wondering what kind of language they're going to get. They already know. Nobody has ever accused Stephen King of having cookie-cutter characters or all his novels sounding the same, but they're all identifiably King.
Voice is a lot more than just the vernacular a character uses. Your story has more than one character in it, and they all need to sound different. But they're all being written by one person, as is all the action around them - the stuff that's
not happening in the characters' heads. That stuff is where Voice lives.