+5V does not come from the Sound Board power supply. That is derived from the power supply pin 5 - that goes to pin 5 on the sound speech board.
+12V is from the sound / speech power supply. If all the other voltages are present on the speech / sound board, then I suspect the +12V regulator (MC7812CK on the speech sound power supply) is faulty.
But given the +5V is dragged low, I would disconnect the speech / sound board and test ALL the voltages at A6/J1. This would rule out the speech / sound power supply.
