If you want something pretty much for free, and you have access to the attic or inside or other side of the walls, you can soundproof with egg cartons. Not the styrofoam kind, but the other kind. I've seen it done...
I work as an acoustic engineer, and this myth about egg boxes has to stop!!
If you stick egg boxes all over your wall they will not help soundproofing at all. You would bee beter of using another layer of plasterboard!!
Egg boxes if they help at all which our expensive sound testing equipment say otherwise then they would reduce the reverberations in the room space and not sound proof. To soundproof you need to build up the density of the wall/floor or ceiling.
You can put large speakers on egg boxes to reduce the unwanted vibrations, but rubber mats (20mm) work infinatly better.
Listen up people, egg boxes were designed to carry eggs, not stop sound!!!
i have been searching for an affordable solution to the sound proofing issue. I discussed this with a saleperson at a local high end store. He suggested carpet on the walls. I then called a carpet store and they thought i was crazy. The carpet salesman thought carpet wouldnt help at all. Hes obviously not an acoustic expert. I found some stick on squares in black and darker black which look nice and its affordable, but in my research the only way to do it cheaply is attach felt covered plywood or mdf. I am building a home theater from scratch in a new house my wife and I just bought. we move in this month. I have major acoustical issues and my solution is velvet drapes everywhere. ceiling to floor drapes
Your carpet salesman is more on the ball than your "hifi" dealer's "acoustic" expertise. I suppose carpeting on the walls could be considered a form of "treatment", and would reduce echo etc, but its "proofing" effects would be negligible.