When we talk about a writer's voice, we mean, in general, the sense we get of the person behind the prose. We "hear" a writer in our head as we read, and what we hear enables us to form a mental image of the writer, thus shaping our response to what he has written.
One aspect of voice is whether the writer speaks to his reader as "I." Another is whether he addresses his reader as "you." The appropriateness of these strategies depends at least in part on the rhetorical conventions that govern the writing situation. But even if the conventions of the rhetorical situation permit "I" or "you" (or "we," for that matter), that is not the only consideration. Some topics, or at least some approaches to some topics, seem to call for more distance between the writer and reader than the chumminess of "I" and "you" permits.
When deciding whether to use "I" or "you," even where they would be permitted, the writer needs to think about how he will come across to his reader, and about whether the response these tactics will evoke is exactly the sort of response he wants to produce.
Another aspect of voice is consistency. Obviously it is inappropriate to switch suddenly between first-person and third-person, but it is also important to maintain voice consistency in terms of levels of distance and formality.
Most people consider the level of formality in a writer's diction and sentence structure to be essentially a matter of style, and since voice is closely related to style, this does make sense. But I would say that as the reader "hears" the author speaking to him, the sense of the person behind the voice is very much affected by the formality or informality of the diction and by the simplicity or complexity of the sentence structure.
Of Aristotle's three rhetorical appeals, logos, pathos, and ethos, ethos is the one most closely related to authorial voice. Logos, or the appeal to reason, depends on the plausibility of the evidence the author marshals to support his position. Pathos depends on his ability to get his reader emotionally involved.
But ethos depends largely on the sense the reader gets of the writer. Is he honest? Can he be trusted? Is he appealing or obnoxious? Does he seem to know what he is talking about, or does he seem tentative, as if he is in over his head? All these things are at least partly carried in the writer's voice. Obviously the writer can present credentials that will enhance his credibility, but all the credentials in the world won't help if your reader doesn't want to believe what you say. And whether or not he wants to believe you depends to a large degree on how he feels about you.
That doesn't mean that the writer needs to be all chummy and familiar with his reader. In some circumstances this would seem so inappropriate that the reader would automatically pull back and disengage from what the writer is trying to say. Imagine meeting your roommate's 80-year-old grandmother for the first time and giving her a big ol' hug while calling her "Nana." This would almost certainly not impress her favorably.
Similarly, in some situations your reader does not want you to be excessively familiar. Once again, it's a matter of rhetorical sensitivity. Just this semester, a student in my Introduction to Poetry class made a serious misstep when he paused every few sentences in his final exam essay to address me with some sort of arch and "clever" comment. His self-consciously ironic chatter was not appropriate for the rhetorical situation, and it hurt his grade on the final.
Another way to think of your voice is as your authorial presence. Some people have a striking presence in face-to-face encounters. You feel comfortable and "safe" with them, or you feel respectful toward them, or you feel charmed and amused by them. As an author, you can produce all of these effects, depending on the rhetorical situation. The trick is not to come across as merely charming and amusing when you need your reader to respond to you as an expert, or to seem too distant and superior when you need your reader to feel comfortable and safe. Often you must blend and balance these effects to produce just the response you want in your reader.
And sometimes--fairly often, really, in academic writing--you need to become transparent, to disappear, like a good waiter who does everything that needs to be done at just the right time, without ever calling attention to himself or intruding on your meal or your conversation.
This sort of "invisible" authorial presence is what I call report voice. Not all essays need to be noticeably "voicey." In fact, as with my poetry student, sometimes too striking a voice is a serious flaw, because in much of the writing students are asked to do in college, they are supposed to adopt a report voice.
But as it happens, the sort of writing a lot of students do in high school and junior high is personal writing, not really academic writing at all. In personal writing, a strong voice is usually a good thing, and in fact, voice is often where my college students shine in English 101. They readily project an engaging authorial presence that makes their essays enjoyable to read, even when, as is often the case, they are marred by other serious writing flaws.
But this quality that is so often a positive trait in their writing for English 101 becomes a significant problem in their writing for many of their other courses, including some of their later English courses, because they can't turn it off when they need to.
So report voice does need to be in a college student's repertoire, and he should be able to use it where necessary, just as he should be able to don a more engaging persona when that is what the rhetorical situation calls for.
As with style, one of the best ways for a student to "hear" his voice is to actually hear his voice--i.e., to read his essay aloud, not just to see what it sounds like, but to see who* he sounds like.
GRADING STANDARD: An important aspect of a student's writing skill is his ability to adopt a voice appropriate to the rhetorical context, and his grade should reflect the appropriateness or inappropriateness of his voice, as well as his skill in handling the voice he has chosen.