Voice is just as it sounds. It's the intonation, word choice, slang, accent, plus the myriad of other sounds that make a character uniquely different. If I were to blindfold you and line up your uncle, your teacher from the third grade, your mother, your boss and your significant other, you would probably be able to tell me who they all were once they opened their mouths to speak. So should it be when writing a novel. But how do you give different characters different voices when there's only one person writing the story? Easy peasy - you get to know your characters as the unique individuals they are!
Let's say I continue with the example of the people you know as they gather at a party. Listen to the way they introduce themselves to others or how they comment on the food, music, and house decor. Pay attention to their character and what that character might be likely to say and how they say it. The schoolteacher might choose to use proper English grammar while your boss or uncle may happen to have a Southern drawl. Your boss might express himself in short, easy to understand sentences or drone on and on in monotone. The point is, they all have different ways of communicating, and drawing on those unique traits is what sets each character apart on the printed page. It's the difference between creating dull, two-dimensional characters and interesting, three-dimensional characters.
A good way to test whether or not your writing is succeeding in creating different voices is to read the dialogue out loud without the 'he said,' 'she said,' tags. Can you tell who is speaking? Or more importantly, can others tell who is talking? The best and most easy-to-read novels are the ones containing very few dialogue tags which allow the characters to speak for themselves. If they are not placed properly in a sentence, the eye tends to trip on these tags. This slows down the reader's pace and in turn ends up frustrating the reader. Instead, find other ways to describe the emotion of the setting and purge the urge to drown your sentences in dialogue tags.
An example:
Two-dimensional voice:
"Where are we going next summer on vacation," asked Marian.
"I was thinking that Cape Cod might be nice," answered Wayne.
"I would rather go someplace warm," said Marian.
"What would you suggest?" asked Wayne.
"I was thinking more along the lines of Hawaii," said Marian.
"Don't you think that's a little out of our budget?" asked Wayne.
"Not if we save up between now and then," answered Marian.
"Aloha, baby," said Wayne.
Three dimensional voice:
"Where do you wanna spend our summer vacation next year?" asked Marian.
"I was thinking Cape Cod might be nice," answered Wayne.
Long pause.
"Something wrong with Cape Cod?"
"No, no. It's just that I'd rather go someplace warm."
He sighed. "Well, what do you suggest?"
"I was thinking more along the lines of Maui."
Silence.
"Hawaii? Don't you think that's a little out of our budget?"
"Not if we start saving our pennies now," she suggested.
"Hawaii, huh?" A smile crept across his face. "Aloha, baby."
