I can't confirm this 100% because I see nowhere in the manual how the speakers are wired , but I'm pretty sure it is also wired the same way as the games in the next WPC board since the audio circuit is identical, but if the sub woofer is unplugged, it will cut sound in all speakers. The 2 speakers in the cabinet are paralleled but in series with the sub woofer. It could be as simple as a disconnected or broken wire to one of the speakers, a broken speaker itself (if the coil of the sub is broken you lose sound every where).

Checking all the voltages and fuses related to the sound board is always the first step if it's not a loose wire.

If all the voltages are there, then you can touch the pins of the amplifier U1 on the sound board (pin 1) and listen for a hum, which would indicated the amplifier itself is good. Beyond that you might need a logic probe with audio or an oscilloscope to test the board further.. U5 x9503 is known to fail too (its the volume control chip) measure its output at pin 5 can tell you if it's working properly.

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