When I put a fresh blade on, I can slice through the foam and it makes 0 noise at all. Completely inaudible. Eventually the blade will start struggling to make a perfect cut (even if sharpened, I use NicPro blades) and around this point you can start to hear it passing through the foam if you listen closely. It sounds like of like paper. Even if I sharpen the blade, it never seems to go back to being completely silent (I use one of those long rounded ones).
It's worth noting I use xacto style craft blades, and not snap-off (ocassionally I use both).