Simply add an audio isolation transformer between the output of the sound module and your external amplifier. An extra boost could be the use of an analog transformer for power your controller board instead of a switching power supply.
I am software guy myself, but it seems that your plan should work as expected. The only thing I would be slightly worried about are those short-circuit resistors, in your configuration they should act as voltage dividers.
If that would be me, I would have tried your circuit long time ago. Are you afraid of a magic smoke?
It seems they have not implemented sigma-delta filter.
I wouldn’t worry about power decoupling at this stage.