One can imagine that, with the 380, the top deck is simply further away from the engines.
True. If you go to the back of the top deck you reach the staircase for the lower deck; as you take each step down you can hear the noise increasing. It is noticeably louder downstairs but only by degrees. I would say that the lower deck on a 380 is still far quieter than a 330. I have a fair few hours on both and the 330 although quieter than the 777 IMO (the 777 at the back is like sitting in a dishwasher, noise cancelling headphones a must) it is still noticeably louder than the 380 especially on take off.