Huh? What exactly does that mean? Funny, but I can only come up with what it doesn’t mean.
It’s not making every one in the story a baritone.
It’s not blathering on and on with backstory. You may achieve depth of story – but that is a tightrope of another kind.
It’s not opening the lid on your characters head and sharing every stinking synapse that fires. I’ve gone down that road and ended with a chapter of giant yawns.
So…? I’ve gone to my humble library shelf of “you can write a novel!” references. No glossary definition. Is this like snapping a picture of sunbeams after a rainbow? If it happens, it’s beautiful. But you’re as surprised as anyone when it does.
Is it like a runner’s high? You’ve heard other people get them, but for you, it remains a myth.
Here’s a shot. Depth of voice means getting into your characters head to reveal pertinent observations about the current setting/situation. It’s controlled wading into murky waters. Watch your step cuz you can’t see the bottom, but step carefully and your toes will gather enough data to keep you going.
Yes, it’s sunbeams after a rainbow. I know when I’ve captured a rich, creative, dynamic snapshot. The scene pops. But I also know when I’ve captured a surficial image of a pretty landscape. Nice, but not on fire. Now if can only figure out the literary “camera settings” to capture the first one every time.