Harry,
Thank you again for your message. TRRS means the 4-conductor audio cable, stereo + mic (like the one used in smartphones).
The highest freq. is around 10k, and the signals are DC coupled at 1.5V via a reference voltage.
I believe I will go for the AD8604, since the footprint is the same and it is more than half the price of the AD8618.
Please let me know if you have any additional comments on this.
Best,
/Elias