Usually audio inputs use protection diodes that shunt into the power supply rails of the device they're protecting. But is this really necessary? Why are TVS diodes not used here? Yes, some are high capacitance. But they have low capacitance parts that are ~3pF or less for DSL modems and such. If a TVS diode can shunt from input pins to the chassis at the entry point of the enclosure, would that not be better?
TVS is for power section of electronics. Small signals that are forwarded to amplifiers would suffer from leakage currents. A high impedance of amplifier input has to be protected by low leakage diodes, less than pA. A JFET transistors are used as diodes, range of 100fA (femto amperes).