wonderfulaudio

Hi, Please see the schematic https://thecarversite.com/manuals/Sunfiremandir/Sunfire Schematic Subwoofer TS-EQ.pdf In page 3, U9 CNV17-1 optocoupler seems to be isolating the high voltage amp from the audio input, but a) How is the optocoupler used for audio as it is not a linear device needed for audio? b) R158 and C144 are connected between low voltage input side to high voltage output side, what are they used for? c) Dont R158 and C144 break the isolation? d) On page 5, BR1 inputs are shorted, this looks like a typo mistake or older manual Thanks and Warm Regards, WA