Take a moment and think of Andy Serkis speaking a line by Gollum.
'Bagginses stole it from us, we hates them, we hates them! gollum, gollum.'
Now read it out in your normal voice. Doesn't sound right, doesn't it?
Andy Serkis's voicing of Gollum is now what people think of when they think of Gollum speaking.
Some of my younger brothers are (while I write this), listening to a recording of the book of The Hobbit.
When you listen to Gandalf speaking (by a different actor to the narrator), it sounds strange. Ian McKellen's playing of Gandalf now defines what we think of as Gandalf.
However, it is not only the voice (and body) acting of these actors that defines the characters, it also stems from J.R.R. Tolkien's amazing work. He defines the character's voices. When you hear – or even read – someone say something, you can tell who it is. For example, look at these quotes:
'What did I tell you? Something's happening!' cried Sam. "The war's going well," said Shagrat; but Gorbag he wasn't so sure. And he was right there too. Things are looking up, Mr. Frodo. Haven't you got some hope now?'
'Now I know what you bear. Bear it still for me a while!' - Aragorn
Now, those were just random quotes from Sam and Aragorn, and so they may not be the best examples, but I hope they are enough to illustrate the point.
It is important to create 'round' characters. But unless they are the main character, the readers cannot normally get 'inside' their minds and see what they are thinking. They have to infer it from their deeds and what they say.
We (should) have all been told that dialogue is an important part of writing. This is part of dialogue. It isn't what they say, though, as much as how they say it. Does your character frequently use contractions? Or longer words? Or are they more unusual and use Yoda-speak?
If all your characters speak in the same way, then readers may have a problem identifying them in dialogue. If they have their own peculiar speech mannerisms, however, then readers will instantly know who is speaking.
Having identifiable characters is an important part of writing. One of the main ways which characters interact with each other is by dialogue. Having vocally identifiable characters will give your writing that extra 'realness' about it. In real life, everyone doesn't speak in exactly the same way.
My main character is based off me. This means that I don't really have to create a speech mannerism for him, I can just write what I would say. This may be an option. However, you need to be careful. If your character lives on the streets as an orphan and you have been used to speaking very proper English, then your speech will not be suitable for him. In my case, my main character has a similar background and language training to me. Even if you do this, you will still need to create other speech mannerisms for other main characters.
You probably don't need to spend a lot of time defining the speech mannerisms of minor characters, unless they have very peculiar speech patterns.
So, do your characters have speech mannerisms? Can you tell who is speaking by what they say? If not, you might want to consider changing their speech so that it reflects how they would realistically speak, based on their background.