This title says it all, basically. I say this about five times a day to clients wanting to know how to soundproof and the question invariably that comes back is: “Why not? Then what the heck is it used for, then?”
It’s not that I’m trying to ruin anyone’s day here—I would love to have foam that could stop sound from going through walls. I would sell a ton of it. But physics is physics, and the fact is that I am not going to recommend a product to someone that has zero chance of meeting their expectations.