As usual with the Konami boards of this era, the SMD capacitors on the custom audio module have leaked resulting in a mute game.
First thing to do is to remove the black paint. In my case it was not too hard, the electrolyte leakage had degraded the paint. I rubbed the concerned areas with a toothbrush:
Then I removed the faulty capacitors:
I've made a small schematic to help you identify the capacitors polarity and value:
As expected once replaced the game works just perfectly fine.