If you put signal wires in a metal conduit, make sure the conduit is grounded. Never run wires in an ungrounded conduit, particularly low-power, unbalanced signal wires, because the ungrounded conduit will actually couple AC noise into the signal wires, acting as a sort of concentrator.

Here's a link to a page about Belden's speaker cable, and it includes a chart of size/power/distance/ for sound systems. This type of product is best, even better than exotic "boutique" cables. For those of you that like to wind your own coils, here's a good program that gives specifications of various magnet wire products.