That's an interesting question. I see it in two parts--what's the speed of propagation of the signal along the wire, and how much time differential in audio can a human detect? Got no answer for you, other than that the speed of propagation is close to the speed of light, so I imagine the difference in length would have to be fairly dramatic. Possibly so dramatic that you'd notice a drop off in sound quality (from the resistance in the extraordinarly long wire) before you'd notice the difference between the channels.
