I installed Pergo flooring a couple of years ago in my home. A layer of foam sheeting,which came with the Pergo, was layed down between the wood subfloor and the Pergo. The sheeting was about 1/8" to 3/16" thick and provided a cusion for the flooring. As you walk on the finished floor, there is a feel of some cusioning but no noise generated throughout the house as you were wondering. I'd be a little careful of putting a foam layer down any thicker. I don't think the floor will perform as well - - even though you may gain some sound deadening for the rooms below. The distortion of the flooring from the person's weight and the additional give of the foam pad may damage the panels and/or cause the tongue and groove joints to seperate.
We've enjoyed the floor. It looks good and is very easy to clean. Simple broom and dustpan for the mostpart with an occasional damp mopping.