The sound board's own self-test plays everything correctly, but in the game some sounds never play. Grounding those sound input lines with the board unplugged from the driver board also gives no sound.

Sound self-test passes but some game sounds never play: bad PIA on the sound board

Cause: The PIA (IC10) on the sound board had failed on the input lines for those sounds, even though the sound board self-test passed.

Checks

• With the schematic, follow the dead sound inputs to the chip they reach and check with a logic probe that the signals arrive.
• Ground each input pin in turn (board unplugged from the driver board): each should make a sound.
• Check the sound board DIP switch settings.

Fix

• Replace the PIA on the sound board, fitting a socket since the original is soldered in.

Systems

• Williams System 6 / 6A

Games

• Firepower

Related tips

• Sounds don't match the game or die mid-game: sagging 7805 on the sound board
• Setting up a sound board for speech
• Sound board jumpers for sound-and-speech games (2K ROM)
• No sound in play or solenoid test, and grounding inputs does nothing
• Sound test works but sounds missing in play
• Sound test fine but game plays the wrong sounds: cracked joint on sound select header
• The sound board is its own computer
• Test the sound board inputs one at a time

Source: Pinball Repair help group!! (Facebook group)