There are several different types of "voice" in a narrative. "The First person is usually subjective: we learn the narrator’s thoughts, feelings, and reactions to events. In first-person objective, however, the narrator tells us only what people said and did, without comment. The second-person mode is rare: You knocked on the door. You went inside. Very few writers feel the need for it, and still fewer use it effectively. In third-person objective, we have no entry to anyone’s thoughts or feelings. The author simply describes, without emotion or editorializing, what the characters say and do. The author’s persona here is almost non-existent. Readers may be unsure whose fate they should care about, but it can be very powerful precisely because it invites the reader to supply the emotion that the persona does not." If the point of view is third-person omniscient, we, as the reader, will be able to determine what all the characters are thinking and feeling.
Voice is the style or personality of the narrative and is one of the 6 traits of writing evaluated in the 6 traits program used in schools all over the country. Although it's easy to tell when a paper has strong voice, it's one of the hardest things to teach students. In my opinion, voice is the trait which incorporates all the other traits to give a narrative personality. A paper with strong voice will have interesting word choice and good sentence fluency; it will use conventions correctly and organize its ideas clearly. With all those traits in place, the author's unique style will emerge.
From an instructional standpoint, teachers are not so much teaching voice as leading students to find their own voices. This is partially accomplished by helping them master the mechanics of writing so that grammatical errors, etc, don't interfere with the expression of their voice. Additionally, I believe voice is best taught by assigning different types of writing to allow students to experiment with different voices. This may be difficult, but is also fun because it invites so much creativity. Possible assignments include rewriting a chapter or event from literature read in class from the perspective of another character, writing personal narratives for different audiences, and trying on different personas and writing on topics related to these personas.
I agree that voice is one of the most difficult ideas to teach, especially to aspiring writers. I try to simplify it for my students until they grasp the concept; I tell them that "voice equals pesonality." The personality that they want to exude in their writing becomes the voice they use. I do try to teach the students to discover their own voices rather than adopt or mimic something else. I ask leading questions such as: What is the speaker's role? emotion? I also ask students to determine the mood of their piece of writing (if it's a personal narrative, what mood they were in when writing it, for example), because mood can lead to voice through word choice and style. To determine the voice of writing that's not your own, I think you must examine the mood, tone and diction in order to reach an appropriate description of the voice.