When my house was built, they pre-wired the walls for 5.1 setup. Unfortunately, the didn't mark where the rear surrounds are. I don't really need the fronts because I"ll run them right from the receiver, but I definitely need the rears. I will probably call them, but I'm afraid the company is going to want to charge me to locate them. Can I find them myself? How? I've gone up in the attic and you can't see where the run down in the wall from there.

Zircon and others make metal finders that can detect copper and other non-magnetics as well as steel in floors and walls. I don't know if they can detect a small copper wire. You might write or call them, http://www.zircon.com

or this-

http://upload.wikimedia.org/wikipedia/en/thumb/4/47/18th_century_dowser.jpg/300px-18th_century_dowser.jpg

Call the contractor. They shouldn't have to 'find' them for you, they should have inspection reports showing where the wires are.

I believe they sell kits for trade contractors that inject a signal on the conductor on one end, and then a handheld signal tracer picks up radiated signal from the wire carrying the injected signal.

I've seen these used but never learned what they're called. Talk to a professional contractor and see if they can offer suggestions.

I imagine the injected signal would be pretty strong, so be sure there are no components on the other end of the wire!