To me, the main thing about the use of voice in teaching, and the quickest way to the source of good writing is getting the self behind words, not holding back, and bringing one's whole heft to bear. This finesses the theoretical, literary questions: is there a self and how many do we have? And how can readers judge whether resonance comes from a good fit with writer or just a good fit with himself as reader. It's a practical question from the writer's point of view, and good readers can see it.
However, this position leads to dangers, such as feedback that is intrusive and inappropriate and condescending or pretentious. "That's not the real you; you sound fake here; you haven't suffered enough." For me, safety and courage are a crucial dialectic. If we emphasize safety too much, and put not enough on courage, we can push people during class, instituting more classroom readings and more public discussions. This has the benefit of breathing life into words.
An Activity: Have a student read something aloud. She should speak so there is meaning/intention in it. Then, write it so that it has "meaning" in it. Have another person explore the opposite strategy: reading/speaking/writing so that their intention is not there and the meaning is hidden. Sometimes the writer holds back and is in a somewhat fishy or sour-sounding relation to the words. These opposing positions reveal one model of "presence" or "life", indicating that there are different ways of writing, just as there are different ways of living.