I've had it perhaps twice, while on medication. The first time it was pretty terrifying - I could hear the voices from far away, then closer, then shouting in my ear and then receding, as if people were walking by. For 16 hours straight, I was exhausted by the end of it as I couldn't sleep. Nothing specific was said - it was like listening to passers-by having conversations. Still, a very haunting experience.
It's most likely a good idea to see a doctor if this happens while not on medication. A friend of mine was committed to a mental hospital for hearing voices. She's now out and doesn't tell anyone she's still hearing them - her solution is just to ignore them. Not an option for everyone, though.