I have a wall that I have soundproofed by adding a few layers of soft caulk (to create small air gaps), foam paneling, and extra layers of drywall. It is now fairly quiet. However, the bottom 1-2 inches were not made thicker because the wall is ~98" tall, I worked from the ceiling down, and I used 8' (96") plywood and foam sheets. I also don't want to put drywall in contact with the floor because of a) thermal expansion and b) possible spills.
Now most of the sound comes through the bottom of the wall which is 1) trim (on the other side), 2) small air gap under the drywall on the other side 3) 4" of a 2x4 that is the bottom of the framing and now 4) A few inches of air underneath my new thicker sound-proof wall.
What is the best material to use to fill this gap before I put the new trim on my side of the wall? I was thinking of small custom-made sand-bags? Or spray foam? Something else?