Re Sound check all voltages at the board.

There is a seperate power supply for this board, but the 5v still comes from the main power supply.

If all voltages are present, do you get a hum through the speakers when you trn the game on?  Turn the volume up is there a hum?  If the hum is there this proves the AMP is working.

It is always a good idea to replace the four electrolitic capacitors on the board as they are well past the design life and can drag the board down.  Another problem is quite often the 2 DAC ICs fail.

Anyway report back, Im sure we can fix it.