Audio port protection discussed in this app note from Littlefuse. Link here (PDF)
Externally generated ESD pulses are introduced through the Audio jacks (headphone, microphone, RCA-type plugs, etc.) and travel through the connector onto the system board. Once on the board, they will propagate down the signal lines toward the integrated circuits(IC). Most ICs are designed with 2,000V of internal ESD protection. It is not uncommon to measure ESD transients in consumer environments over 8,000V.Without sufficient protection, the Audio system can experience visual distortion or corrupted data. In addition, the IC could be permanently damaged rendering the whole system inoperable.