Sound systems can range in scale from being simple to very extensive. Generally the more components that are earthed in the system, the more likely an earth loop will develop. It forms when there is more than one path for the audio ground or shield signal to flow through. For example, if a system consists of a preamplifier and power amplifier connected by RCA interconnects, and both the power amp and preamp are earthed to the mains, there are two paths for the ground signal to flow. One through the shield and return wire in the interconnect, and the other via the mains earth in the power board or house wiring and the two equipment earth wires.
Here is where the GND Defender can be of assistance. By inserting it into the earth system of the second (or more) low-resistance earthed component, you may reduce the effect of the earth loop, evidenced by a reduction in hum or buzz
