How do the cabin noise levels of the A380 and the 777 300 ER compare?
From a luxury round the world flight on several different models of aircraft on several different airlines, a traveler used the exact same sound meter at his seat in either business or first class to measure ambient noise level during cruise. 

Here is a chart I made of his measurements, including carrier and age of aircraft. As you can see, long haul aircraft are quieter than short. Carrier and age of aircraft also seem to make a difference. 

For this set of measurements, the b773 is 3db (or 25%) louder than the a380.