Experimenting more, the sound sometimes works and sometimes doesn’t.

By going through the diagnostics in the Williams 1980 Solid State reference manual, it appears that the problem is on the speech board. Symptoms:

• if I jump the negative pole of C29 to 10J4 pin 1 and press the self-test, I get one cycle of electronic sounds
• It says that the likely problem is address or data connection to the speech module, or that IC6 is faulty
• Checking Table 13, which is for a slightly different problem, it says to check for data activity on sound board IC 10 pin 39 and speech module IC1 pin 12 or 13 (depending on version). Neither of those pins have activity.
• I checked the data and address lines on IC6 on the sound board and they all seem ok, so that’s getting
• Swapping IC10 from Blackout solved the problem. Both speech and sound work. So I’ll order the part.