Voice, like art, is one of those things that, being hard to define, often falls into the category of “I’ll know it when I see it.” It’s a quality that writers strive for and editors look for, precisely because it’s so hard to accomplish.
There are two kinds of voice; authorial voice, which is what writers bring to their overall body of work, and character voice, which is how each individual character sounds to the reader.
One of the great advantages of Deep POV is that, if your characters are well developed, their voices will pervade the narrative. In Deep POV, the main voices one notices are those of the characters.
The flip side of that is that one of the drawbacks of omniscient POV is that the narrative voice can detract from that of the characters. One of the issues we frequently see in amateur manuscripts is characters who all sound alike, regardless of their age or level of education. This problem is magnified if the characters also sound like the narrator.
There are several elements of voice, too many to go into in this brief space, but chief among them are diction and sentence structure. A young man will speak differently from an old one, and a poorly educated fellow will use different grammatical patterns than a highly educated one. But even when the differences between characters are not great, one should be able to see distinctions between them.
I like to use Charles Dickens’s A Christmas Carol as an example because you can go get a copy on Kindle for free. The narrator takes on an almost snarky tone at times.
He lived in chambers which had once belonged to his deceased partner. They were a gloomy suite of rooms, in a lowering pile of building up a yard, where it had so little business to be, that one could scarcely help fancying it must have run there when it was a young house, playing at hide-and-seek with other houses, and forgotten the way out again.
That’s one voice. You can also see voice contrast between Ebenezer Scrooge and the fellows who come collecting donations for the poor.
“Under the impression that they scarcely furnish Christian cheer of mind or body to the multitude,” returned the gentleman, “a few of us are endeavouring to raise a fund to buy the Poor some meat and drink, and means of warmth. We choose this time, because it is a time, of all others, when Want is keenly felt, and Abundance rejoices. What shall I put you down for?”
“Nothing!” Scrooge replied.
“You wish to be anonymous?”
“I wish to be left alone,” said Scrooge. “Since you ask me what I wish, gentlemen, that is my answer. I don’t make merry myself at Christmas and I can’t afford to make idle people merry.”
Scrooge and his interlocutor are of about the same age and social class, but you can see that apart from the friendly attitude of the gentleman and the ornery one of Scrooge, there’s also a difference in word choices and tone, with the gentleman using fluid phrases like “abundance rejoices” and Scrooge using terse ones like “idle people.”
Whatever POV you choose, your characters should be this well delineated. But when you are using omniscient POV, it is especially important that your characters sound like themselves, and not like the narrator.