Two wires, positive and negative, together reduce electromagnetic and capacitive noise pickup. They also reduce noise radiated from these wires. It is because noise is being picked-up (or radiated) by both wires that are at opposite polarities thus cancelling. Twisting wires greatly helps because both wires are exposed more evenly to external fields. Speaker output is also (in most cases) an input to amplifier and wire acts as antenna for noise.
Twisting wires reduces inductance, as Al stated, but also increases capacitance. In case of the speaker wire capacitance does not play as much role as inductance. In interconnects it is just the opposite but twisting is still used to reduce noise pickup.
Twisting wires reduces inductance, as Al stated, but also increases capacitance. In case of the speaker wire capacitance does not play as much role as inductance. In interconnects it is just the opposite but twisting is still used to reduce noise pickup.