I did this once before too. Found on Mike's Arcade:
Problem: All sounds are present, but very low volume.
This board took a hit from the monitor sound amp being hooked up backwards 1. The large 3 pin connectors were put on incorrectly. When the game was powered on, the board smoked about 5 parts, R1, R38, R28, R29, C12 and Q1. The most of them actually flamed and all were toasted. Also the traces under R29 and R29 were burned, as well as the pads that R1 were soldered to were destroyed. After fixing all those parts, the sound was still faint.
Solution: R1, R38, R28, R29, C12 and Q1 and repaired many toasted traces
This also tends to knock out the green from the display. See the next log entry for details.
Problem: Green color absent from the display.
This seems to be a common occurrence if the problem above has occurred.
Solution: replace bad transistor Q14, emitter was stuck low.