I've had this happen any number of times, always while I'm very tired or falling asleep. It doesn't sound like an actual person speaking out loud, but it's sort of like a strong mental impression of someone speaking, if that makes any sense. With me, it's usually an incoherent murmuring. Reminds me of a line from the Rainbow Connection: "Have you been half asleep, and have you heard voices?" To my knowledge, it's perfectly normal. You don't think it's dreaming because you're still partially conscious, but I'm pretty sure it's basically just the beginnings of a dream.