When I lived in a small apartment in the San Francisco Bay Area, I bought a bunch (~9) of bargain-bin flat king-sized sheets (the flat sheets only, not the fitted ones) and copper piping cut at the store to fit the lengths I needed and closet supports. Some sheets come with a fold at one end that will work to slide a pipe through. Others, you can have it customized for very little money at a tailor shop. Alternately, you can buy clips to hang the sheets as curtains without any modification. I found that having them tailored, so I could wash them frequently, worked best for me. For one especially long wall, I bought a hook for a middle support. Some of these curtains were used to separate rooms (the living room from the dining room, the dining room from the kitchen) but others were hung flat against a wall solely for decoration and sound-proofing (between the master bedroom and kid's bedroom). I think I spent roughly $100.00 for the whole thing, plus a few bucks more later when I took them to a tailor to modify them because I was sick of dealing with the clips.

If you do this to divide spaces, you do need to consider that air flow may be an issue. But if hung flat against an exterior wall just to block out noise (and assuming the wall itself has no air vents), this shouldn't be an issue.