Repinned and replaced connector 10J4, sound board remote volume. Didn’t help.

Debugging the sound board:

• fuses are good
• input power is good
• on-board regulator and voltage converter is good; test points all have approximately correct voltages
• checking the speaker output with the oscilloscope shows nothing
• same for the output of IC1

Reading further, the sound board signal gets sent to the speech board for mixing with speech and then comes back for output.

Continued!