I'm sure there are quite a few posters here who lose their voices no matter the situation, but for the crowd at large what really affects crowd noise is how much the crowd thinks they need to be loud. Division games, close games, games against good offenses, etc. Playoff games have been the loudest in my experience, but also early season divisional games especially when the weather has been mild.
Yesterday was a cold and rainy outdoor game against a team from another conference that everybody thought we would beat handily. We were never behind, and everybody felt that the Jets offense sucked. Is that an excuse? Maybe not, but it shouldn't be surprising.
If you are looking for the loudest experience come to the 49ers game and you'll get way more noise, especially if the game is close and the crowd thinks their contribution will have playoff implications.