As long as you can hear it, then I think you have reached the minimum level. If there was a minimum volume level, I think if really would depend on the size of your room and what kind of sound revereberation is in the room. You really need to make sure your room is setup for sound wave control.
Anyway, that said, I don't think there is a minmum volume.
As with any surround sound be it 5.1, 6.1 or 7.1, is there such a thing as a minimum volume level to enjoy the full splendour of sound or is this just another silly question?