I believe indoor stadiums increase the sound over open air stadiums because of the acoustical advantage of all sound being captured. I was just at the Vikings/Broncos game (which was my first game ever at that stadium) and it was the loudest arena I have every been in. I have been in the Rose Bowl, Qualcom (San Diego), Heinz Field and none of them compared even remotely to the noise. My ear drums were vibrating at many points of the game. i do not believe the indoor stadium took the life or the sound out of the crowd.
Also when it's 10 degrees and snowing and the crowd are in thick gloves, face shields, and heavy coats that deadens the crowd and the noise.