"The voice" is a rather apt metaphor for the way that a lot of people think. You probably subvocalize, which is a fancy word which is used to describe "reading aloud" in your head. Not everyone does this, so the metaphor of "the voice" is alien to certain people. I know an autistic man who "thinks" with images and mathematical equations moreso than with words. When I asked him if he subvocalizes, he said that he was unable to answer the question because he wasn't sure if he does this normally but he was certain that he was doing it immediately after I asked him, which is the telltale sign that he doesn't do it normally.
There's a rather interesting essay published in book format by the late Julian Jaynes called "The Origin of Consciousness in the Breakdown of the Bicameral Mind" which puts forth the theory that, as recently as three thousand years ago, it wasn't uncommon at all for people to literally hear voices in their heads in the same way that schizophrenics do, and that these voices were the method by which the right brain of humans communicated with the left brain and thereby the mechanism by which people made decisions. The book further states that there are perfectly sane people living today who still hear such voices, and who might take advice from them. It's quite an interesting read and I highly recommend it. It's only too bad that the author died before he could write his follow-up. In answer to your question, it might be our sanity, but more likely it's our subconscious letting us in on little pieces of what it's been thinking about and trying to help us make wise decisions without overwhelming us with all of the information that our brains have to process.
I don't literally hear voices, but I do have "a voice." The way I explained it to my autistic friend is that I think in silent conversations with myself. This is especially apparent when I've been drinking, during which time I sometimes write lengthy messages to other people which amount to conversations directed at myself rather than at them. For the large part I try to take its advice, but I find it difficult to bring it on in the long term, meaning that my voice often helps me in short-term decision-making but that I struggle to follow along with it, much to its dismay.
Since this is /lit/ I find it necessary to link this conversation to writing, in which case I must say I don't listen to "the voice" when I'm writing. I find it distracting. I silence it by listening to music. I imagine it finds this very annoying.