I believe the best way is to do offset studs with some form of interior insulation. I believe Roxul now sells a batt that is solely for soundproofing, and is pretty economical.
As for the wall hanging idea... it is a good idea but you need to actually fill them with something to have a dramatic effect. Fiberglass insulation works well...
My own approach over frames was to buy some jute-like blinds from IKEA... insulate underneath.. and blue-tak the edges down. Minimal fuss... looks nice... and inexpensive.
acg.
