Most writing manuals advise writers to keep the tone of their stories consistent. In itself, this is good advice. But what these guides usually mean is that writers often err in how they vary tone, so it can be best, especially for the emerging writer, to play it safe and keep a story's tone consistent. Yet, like all manmade rules, this one can be broken. Here's when and how.
First, let's define tone. It can help to think of tone in writing like the tone of your voice. With virtually limitless variations, you can convey a range of meanings and emotions — sarcasm, joy, sadness. And you can convey degree of meaning and emotion with volume. The same is true of writing.
To hone the definition, consider that tone helps create mood, per James Scott Bell in Revision & Self-Editing. A more complete definition comes from Noah Lukeman in The First Five Pages. As he points out, the distinction between sound and tone is subtle. Sound has to do with sentence structure (flow and rhythm), whereas tone "is the voice behind the work." As to how tone relates to style and voice, "A writer's style covers all of his work, while he may alter his narrative voice from project to project to suit each one. Voice is subordinate to style," as Peter Selgin pointed out on this blog.
So how do writers err when it comes to tone? Most often, like point of view shifts, writers vary the tone of a piece in the wrong place — like in the middle of a scene, early in the development of a character or in the overall story. Since tone creates mood, you can gradually vary the tone in a scene to increase suspense, or use it to show character development. The key in both instances is timing. In a scene or character, it's best to change tone gradually to show progression and avoid jarring the reader. In a story as a whole, tone has a lot to do with genre (think of TV shows with categories like comedy, drama, mystery). While many stories cross over from one genre to another, most maintain a consistent tone throughout.
This doesn't mean tone should never vary. Some examples where this can work is to distinguish one character from another, to provide nuances in scenes, and to add depth and breadth to the story. Yet, as with tone of voice, less is often more. So be attentive to "sound" of your writing when you go back and read a piece, and especially as you return to edit.
Resource: Find Your Creative Muse