double then triple check that all the 4 wire cables are on in the correct spots and no wires are pulled out. Those ones send the sound from the MPU to the sound board and out the sound board to the volume knob then from the amp to the speakers. One broken wire going to the volume trip pot will cut ALL the sound in the game, same if one of the speaker wires got yanked.

So now double check that all the voltages are making it to the sound board. 5, 12 and NEGATIVE 12. That negative 12 is used for the speech chips and the amplifiers and goes to both the background sound board AND the MPU board sound. Maybe yanked the wire out of the power supply end of the connector.
Semi-related I have seen multiple times on the 11C version of the power supply where the 2 pins that supply -12 volts one has the trace on the front of the board under the plastic part of the molex connector have a cold solder joint that wasn't evident from the back of the board that caused a loss of sounds. So not applicable in exactly this situation, but a loss of -12 will cause no sounds no matter where the -12 is lost.