ESD and accidental line-in protection

Hello, thanks for the questions.

Though I cannot give a specific recommendation for ESD mitigation for your application, I will point out that a common ESD mitigation technique is to use a TVS (transient voltage suppression) diode placed closely to the audio jack with a short path to ground. TVS diodes react more quickly to voltage transients than Zener diodes. This may prove effective for your situation.

For your second question, the MAX9728A datasheet lists the absolute max rating for the OUTL and OUTR as described below (found on page 2 of datasheet):

"Note 1: OUTR and OUTL should be limited to no more than 9V above SVSS, or above VDD + 0.3V, whichever limits first.

Note 2: OUTR and OUTL should be limited to no more than 9V below VDD, or below SVSS - 0.3V, whichever limits first."

If you find that an accidental insertion may exceed those limits, you could try a low forward voltage diode clamp. There are other mitigation techniques that may work, but this is a common one.