I think this voice is part of The Dark Lord's essence/spirit that was transferred to Harry on the night of the attack on his parents and himself. I think that the part of You-Know-Who's mind that is in Harry is the thing that gace him the scar and is also the thing that causes him pain when he is near The Dark Lord - as if the part in Harry's head is trying to escape and rejoin with You-Know-Who.
I expect a part of one of the forthcoming books will reveal lthat Harry had an imiganry childhood friend called Tom Riddle - name coming from the voice inside his head. We'll learn that Tom Riddle has been talking to Harry since an early age and would also be the same part of The Dark Lord's spirit that has helped Harry do his Dr. Doolittle bit and talk with the snakes.
That's what I think anyways....I could be wrong.