Microphone level signals of a few milli-volts are more sensitive to external interference. A very common trick is used nowadays in order to improve signal to noise ratio. The signal level is elevated in order to elevate the signal to noise ratio. This trick is based on the assumption that interference is constant. Today, signal levels of 6-30 Volts, travelling on balanced audio wire systems, are very common.
Cables impose specific impedance in a signal path.
Sometimes, the resistance part of this compound impedance is too large and creates a voltage divider with the load impedance. The result is signal level deterioration. Active amplifying circuitry compensates for signal level changes where signal gain, either at the receiver end or, preferably, at the transmitter end, can be fine-tuned by the user, using internal or external controls. In some cases, some specific frequency bands within the audio signal spectrum are attenuated due to cable capacitance. It is usually the high frequency range which is effected.