I'm beginning to suspect that the nearby high voltage transmission lines may be an issue. You will definitely want to use steel conduit wherever possible, but to minimize ground loops try to ground the conduit at only one end wherever possible. If you have concrete or steel structure that the conduit and boxes are fastened to, single-point grounding may not be possible.
The zip cord that you describe probably isn't twisted pair. It may be picking up induced current from the transmission lines which then energizes the voice coils in the speaker. Using twisted pair speaker cable may be a benefit.
For grounding issues, I'd suggest contacting
Mike Sokol. He's a member of these forums and has done a considerable amount of research into grounding, ground loops, and reducing noise in systems. (Click on his name to view his profile or send him a message.)