When is unequal, equal?

I wouldn't be so sure of the speed of light thing. While it is true, it is not the speed of the electricity traveling through the wires that is a concern, but the resistance, capacitance, and inductance, which are functions of length.

A difference of 8 feet will not make an audible difference. This is easily shown with the math, but without the boring details:

Assuming the speakers have a 90 dB/2.83V-m sensitivity at an 8-ohm nominal impedance (1 watt) and that the speaker cables are 14 ft and 6 ft of #12 copper, the voltage drop to the furthest speaker will result in -0.035 dB. If you're 8-feet away from the speakers, you can compensate for this by moving that speaker a quarter of an inch forward. I didn't take capacitance and inductance into account - I don't think it will make a difference, but I could be proven wrong.

Good work, GS. Every time you move your head while listening, you have a greater effect on the relative weight and timing of the sound reaching your ears from your two speakers than anything caused by a modest difference in cable lengths. In other words, if you think equal cable lengths matter that much, your listening chair should include a head-vise.

Also, Pabelson, the 'moving head' effect applies to all of the other tweaks that seemingly change the sound. All of those 'differences' if even true, are swamped by moving one's head (ears) by millimetres!
Bob P.

The key point of Gs' response was that he didn't take capacitance and inductance into account. Since these are 2 of the most important factors, nothing has really been proven. If you don't believe cables change the sound, then you also should believe that length also won't. However, if you accept the notion that cables do affect the sound, then 2x the cable length will cause 2x the sonic trait of that cable, minus the effect of the terminations.