Technically, you can connect an amp and a speaker with an instrument cable and get sound from the speaker. However, the very small-gauge center wire in an instrument cable presents significant impedance to the current flow between the amp and speaker. At best, it will result in a massive loss of power and volume. At worst, the cable could heat up to the point it melts away its insulation and short-circuits to the shield, or the wire could simply melt into an open-circuit. Either scenario may result in severe damage to your amp and/or speakers. In addition, the electrical properties coaxial cables may have enough capacitance to trigger the protection circuits in some amps, which can result in untimely shut-downs. Speaker cables are specifically designed to avoid these problems, with thick, parallel (instead of coaxial) cables.