Balancing is when the same signal is flipped in phase, sent down the wire, and then flipped back into phase at the other end. Then any noise picked up gets canceled out.
So to do this you need two hot wires carrying signal ("sound" is essentially variable AC) and one ground for shielding and well ground. One carries the in-phase, the other carries the out of phase.
So a Headphone extension cable will get the signal there, but there is an additional factor to consider.
Inside what is normally used for balanced cables the wires are twisted. Electrically since the signals are out of phase and twisted, they actually boost each other with the leakage through the insulation. Also they tend to be a heavier gauge, better insulation, better shielding, etc.
Headphone cable is not typically twisted since the right and left channels are distinct and unique. So what good headphone cable is two independent shielded cables wrapped with an outer shell. Think of it as two guitar cables taped together. The cable also tends to be super thin, and generally crummy. I have seen some insulation that was paint.
Now, the primary difference between speaker and instrument cable is the gauge and shielding. Although I have seen shielded speaker cables too. A shield is just the ground wrapping round the signal wire.
An instrument cable has the conductor in the center and a ground shield of spiral or braided wires for maximum coverage and reliability. This essentially makes a capacitor as well. Also since the signals are lower, the wire tends to be thinner.
Speaker cable is typically not shielded and use heavier gauge wire. Since the voltages going through are so much higher, it doesn't typically need shielding since RF and stray cycle noise is too small to even make a difference.
As far is why it is bad to use an Instrument cable as a speaker cable, my understanding was that the capacitance was bad for speaker cables. Causing shelf roll-offs and generating more heat. Of course there is the wire gauge issue which will also cause it to heat up and melt and fail.
