What material could I use to line curtains over a door to make it more soundproof?

What kind of door? If it's a good door, it may be that the weatherstripping around the door needs to be replaced. This will make your home much more energy efficient. Your local utility company may provide free energy audits and have good suggestions. Otherwise, I ran across this webpage that gave suggestions for curtain materials and liners that serve different purposes: http://www.hutchal.clara.net/curtains/fabrics.htm Roclon appears to make some which you can get just about anywhere that sells fabric, including Walmart. Or try Googling "soundproof drapery lining" or "acoustical drapery" to find something that fits your budget and needs. Soundproofing.org's website had this to say: Install window "plugs". Install gaskets around existing windows and doors; install storm windows and doors, replace hollow core or paneled entrance doors with solid core doors. ... more