I would probably program a fixed volume on the amplifier and regulate the rest from the audio codec side so you have normal software controls, but it also depends on the output/input drive levels of your amplifier circuit. As for four channel audio, do you need 4 different channels in your car? Usually it's only two with left front/back and right front/back so you just buffer the output from the Pine and feed that to two inputs? As for the control signals, that's up to how you want to use your amp and has basically nothing to do with the Pine... I would route them to some GPIO and set sane default logic levels... Maybe also add isolation for the audio path / power supply to avoid ground loops... Since you control some things via I2C you probably need to develop a 'fake' soundcard driver so you have a mixer device to control things....
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