When you sit down to write a story you probably already have a character in mind for it. Male or female, young or old, nice and friendly or nasty and sharp-tongued, this person can be anything. You need to know who this person is, inside and out in order to proceed with the story. You need to decide all that before you can develop a distinct voice for your character.
How to develop distinct voices for characters is second nature to some writers. They can actually "hear" their characters in their heads as they write and develop them and their voices as the story progresses. Many writers claim to be like J.K. Rowling who insists Harry Potter walked into her head fully formed. Lucky them! If your characters do you the great service of doing a "Harry Potter" just strolling into your mind intact then half your work at developing a distinct voice is done for you before you even get to your computer or to pen and paper.
Others, however, don't have it that easy and must use creative means to develop a distinct voice, one never heard before. A tall order, to be sure, but if your story is to stand out amongst the deluge of stories and characters out there, you must try to make your characters as unique as possible. To do this you need to create and develop your character before you can assign a voice to him/her. Doing the following will help you in developing distinct voices for your characters.
Nationality.
Your character will sound completely different if he is born and raised in Italy or if she is from Egypt and raised in Russia. Dialects, accents, inflections and culture all come into play to make this person real. Narrow it all down to a particular place in the world, or out of it if you're into Sci-Fi, and use that to create the character's unique voice.
Personality.
You need to know what makes this person tick. Is he a psychopath or meek and mild, friendly or stand-offish, angry at the world, happy as a lark or defeated and sullen? Is she sexually promiscuous or a prude, shy or flamboyant, quick-witted or dumb as a box of cornflakes? All these thing will give the character a distinctly different voice.
Profession.
Your character has to have a profession. If you make her a famous brain surgeon you can be certain she's a bit brusque, forceful and possibly arrogant in her speech and mannerisms. She doesn't have time for idle chit-chat, you see, and needs to get to the heart of the matter swiftly and concisely. On the other hand if she is a mousey, bored housewife that likes to break up the monotony of her humdrum life by shoplifting bubble gum at the checkout line at the supermarket she'll have a whole different way of talking, won't she?
Plot.
As every writer should know, the trick to a story being interesting is the situation in which the character finds himself. If a contract killer suddenly finds he grew a conscience he might be in a bit of a dilemma when he can't kill anymore without feeling badly about it. His voice therefore, and yes, that even includes his thoughts, will be of two minds. He wants to stay focused and tough but his feeling are interfering. What a mess! And what a good chance to deal with two distinct voices in one character, his voice to the world, careless, cold and detached and his inner voice, confused, guilty and miserable, to which only the reader will be privy. What fun!
Outside Forces.
Everyone of us is a part of a whole and so are our characters unless they live in a vacuum, and what would be the fun in that? Therefore your character has a family, group of friends, coworkers and complete strangers which will cause some friction and with that action and reaction. With this reaction we find a distinct voice, a certain way he will act, speak and think. Outside forces if abrasive enough can also turn a character completely on his head, adding to or changing his view of life, his reactions, his personality and with that his voice.
Mood.
The story itself has to have a mood. Whether a , fantasy, thriller, romance, satire or sci-fi the story has to provoke a mood in the reader. Serious, chilling, funny, inspiring, sad, touching or a heartbreaking mood each will make your character a bit different and with it her voice. Her voice will also depend on what you wish to accomplish and how, either with sarcasm, humor or seriousness. You have to know how you want your character to feel and act.
These are some surefire ways to bring life into your characters and hopefully give them distinct voices. But if you still find it difficult to make your characters voices distinct simply close your mouth and listen to the world around you. Eavesdropping is the greatest and easiest way to develop distinct voices for your characters simply by watching and listening to people on the street. Folks arguing about politic as they pass by on the way to work, kids at the bus stop eager or dreading to get to school, a harried mother of three kids all under the age five muttering to herself and plotting the death of her very amorous husband who got her into that mess or the sweethearts oh-so-in-love cooing softly to each other over cups of coffee at Starbucks.
Every person out there has his or her very own voice and there's no rule saying you can't "borrow" them for your own fictional characters exactly as is or exaggerated. Then there are the characters we base on people we know and love ( or hate) possibly taking some liberties to make them more fascinating, more terrifying, wittier, dumber or more amusing. If your grandfather is a character already, as your grandmother has probably said a hundred times, what harm is there in "stealing" his distinct, gravelly voice for your story? None whatsoever and no one need know. That is until grandpa recognizes himself in one of your stories and is not too happy about it. You're on your own then.