Well they claim to put special fittings in the body and frame (don't know if they did or not). But I can testify that it was indeed quieter. I'd suggest you try and find a Prius with the soundproofing and one without. That's what I did, and I could notice a big difference. But I'm blessed to have pretty good hearing so small sounds stand out for me.

Currently atempting a KiloTank WITH AC (1000 mile tank)
Houston Hybrid and Hypermilers Club <106mpg.com>

Best commute RT = 27.5mi @ 90.5 MPG (sg2)
Best (non-trivial) tank = 665.5mi @ 85.7 MPG (fcd)
MPG Centurion - Hybridfest 2007 - Prius II - 26mi @ 106 MPG (sg2)
Dan <11011011> @ CleanMPG.com