"I just want a story with a standout voice"
"Overall, I just want a good, new, fresh voice."
"Send me your amazing voices."
"I want a diverse voice."
If it doesn't because you're just starting to venture into writing, just you wait. You'll be barraged with requests/discussions of voice forever more. If you're a querying writer, you probably see agents and editors say phrases like those above roughly always. I'd wager they say it more than "hello" or "good morning." When I met Eric Smith--my agent--in person, he held out his hand and said, "Good to see you, give me a fresh voice." (exaggeration. He actually said something like, "Hey man, so glad you made it. We've gotta take a selfie now.")
When I first started querying, my reaction to "give me a good voice" was typically, "WHAT DOES THAT EVEN MEAN?" The only context in which "give me a new voice" made sense to me was that, somewhere, there was a fantasy creature locals called "The Silence" who steals the vocal chords/voice of its prey. In this context, there's also a girl protagonist, probably adept with a bow, who finds herself in a forest because she's the sole provider of food for her siblings. She knocks back an arrow to make a kill--a bird of some some sort, which she bemoans because that means in order for all her siblings to eat, she has to go hungry-- and she hears her long dead father's voice behind her. She lets out a breath she didn't know she was holding, and then finishes what her father couldn't, letting the arrow go straight into The silence. Finally putting the voice-stealing beast to rest.
Sorry. That escalated quickly. All that to say, if you're clueless about voice, I'm going to attempt to shed some light on it
1. Think About Vocabulary
Conversing while traveling across the US will yield millions of different words and phrases. Each region has it's own slang and vocabulary. The mid-west uses the phrase " too yet" as in, "we've still got to go to the grocery store, too yet." The deep south says "ten" like "tee-en," somehow turning it into a two syllable word. The north still uses the word "wicked" as in, "that movie was wicked." I've never heard someone from Wisconsin say, "that movie was wicked" and I've never heard someone from California pronounce "ten" with two syllables.
All of these differences make up a "voice," and your character will/should be effected by this.
2. Think About Yourself
I have a friend that uses the term, "I'm on the struggle bus" frequently. Does everyone say this? Nope. This is her phrase. Part of her "brand" if you will. We all have words we favor over others. One person might say, "I'm going to eat this sandwich" where another might say "I'm going to put this food in my stomach." If your character doesn't have any words or phrases that are unique to her/him, then she probably doesn't have a voice, she just exists. Existing is fine if the character calls for it. If the point of the character is to be uninteresting, then give her a voice that makes her "just exist." However, when someone asks for a "fresh voice" they don't want someone who's just existing. I'll even venture to say your main character should never "just exist" in terms of voice.
Another note on this. If you're not developing each character outside of how you talk, then all of your voices will sound similar. You should be able to see bits of yourself in every character you write, but you shouldn't be able hear yourself in every character you write.
3. Think About Context
This ties into #1 a little bit, but I wanted to be more specific with the details. Where you live is just as important to voice as how you live. Are the main character's parents academic types? If so, then he/she will probably have a bigger vocabulary because he/she grew up having academic discussion. Is the MC religious, a hipster weed-smoking vegan who only eats free-range donuts? This will effect their vocabulary as well. What economic class are they in? Are they creative or analytic? Age? Race? Friend group? All of this matters to voice. Diversity is important here. I, white male Dave, cannot assume that I sound the same as a character of a different gender and ethnicity.
A while back, I took a quiz that guessed my age by the words I used. It was a few years off, but not many. How could this quiz peg me so well? Because of the concept of voice. It took prevalent words and slang used in specific years (yes, even decades have their own voice) and made the user choose between them.
Knowing the context of your characters isn't optional, it's the foundation to successful writing.
Conclusion, Ending, Goodbyes
People most frequently use the phrase "world building" in reference to fantasy and science fiction, but it's important to include world building in ALL genres. Why? Because so many things make up a world, and all of those things comprise the context of a character. So, if world building equals context, and context leads to voice, then we need to know our world to know our voice.
Voice is the sum of the character's everything. This includes experiences, world view, upbringing, friends, family, personality, creativity, habits, hobbies, hopes, dreams, desires, and so on. All of these things effect how we think, and, therefore, our language.
Alright, I'm done. I really hope this was helpful! If you have any questions or comments, hop over to the Twitter and hit me up: @daveconnis.
Go forth and understand voice, pupil.
P.S. See that last line? That's a good example of voice.