The difference between garden variety electrical cord and expensive signal cable is revealed in the OP's original question. His question is based on the presumption that electrical cables and signal cables have the same job--to conduct electricity. But that's not the case. Electrical cables only have to conduct electricity in the form of watts from point A to point B, while voltage remains relatively constant.
Audio signal cables (interconnects and speaker cables) are primarily designed to transmit the accurate values (in rise time, amplitude, and waveform) of voltage fluctuations--signals--that represent everything from the sharp transient attack and low fundamental of an orchestral bass drum thwack to the finest, subtlest timbre-defining overtones of the violin and cymbals in the 12-20Khz region. Electrical cord has no such mandate. You can use electrical cord for speaker cable and sound certainly comes out at the other end, but it's not optimized for accurate signal transfer.
Audio signal cables (interconnects and speaker cables) are primarily designed to transmit the accurate values (in rise time, amplitude, and waveform) of voltage fluctuations--signals--that represent everything from the sharp transient attack and low fundamental of an orchestral bass drum thwack to the finest, subtlest timbre-defining overtones of the violin and cymbals in the 12-20Khz region. Electrical cord has no such mandate. You can use electrical cord for speaker cable and sound certainly comes out at the other end, but it's not optimized for accurate signal transfer.