The makers of the video were commenting that with the duct the kopter was much more quiet. Wondering why duct would make it considerably less noisy. Anyway - I too found the whole machine very interesting and fresh.
If I remember correctly (from the days when I was flying Cessna 172's) most of the noise from a propeller comes from the tip of the blade slicing through the air. My thought is that the ducting is blocking the noise of the propeller and that is why it is a little more quiet.