Joystiq takes on the claim with a rather deep scientific analysis of the new game's guitar and drum pads. Using a microphone and a laptop to measure the decibel increase across three different modes of play (light, medium, and hard key presses and drum strikes), they came to the conclusion that yes, in fact, the instruments are quieter.
Of the drums:
"The largest difference between the two drums was observed at the medium drum strike intensity, which was designed to emulate normal play conditions. While the difference in loudness is definitely noticeable, the Rock Band 2 drums did still have a noticeable "thwacking" sound at this level."
Of the guitar:
At all intensity levels tested, the Rock Band 2 guitar frets were noticeably quieter than the original Rock Band guitar. The original guitar frets had a distinct "springy" sound when popping back up, a sound that was almost entirely absent in the Rock Band 2 guitar. It should also be noted that the spring for the strummer seems significantly quieter on the Rock Band 2 guitar, though the extent of the difference was not tested.
And in summary:
Rock Band 2 instruments are indeed quieter than their Rock Band counterparts under normal play conditions. Additionally, the added springiness of the Rock Band 2 drums and the quieter springs in the Rock Band 2 guitar make the new instruments much easier to play lightly and, thus, more quietly.