This is something that I’ve seen asked so many times, and I thought I’d throw my hat into the ring (as though the ring wasn’t already overflowing!). Voice (and by extension, style) is such an intangible concept, so hard to explain beyond, “Something compelling about the writing that makes me want to read on.”
I think the trouble comes precisely when we attempt to define what makes voice. It’s unique to each individual, and though it may be very different between our characters, it’s still something that’s very much you. So, how do you go about finding it if you don’t even know what you’re looking for?