The C2 capacitor could be the main problem to the volume issue,If everything else is working properly like the menu buttons ect.This 100 mfd cap seems to fail causing the volume to go to the max.If this capacitor fails/leaking out then it is no longer good and will need to be changed out.The leak from this capacitor is somewhat corrosive and can ruin the circuit board traces.Here's a pic showing the capacitor and yes that white substance is the leak.(EDIT-I posted this for future reference the OP found it was a wiring issue but if all else fails this is the problem C2 capacitor)