It sounds like the game's MPU randomly locks up with the sound board connected. Try to determine why... Get it booting and safely shake the crap out of the MPU/head/sound board banging/tapping everywhere. It could help point to a bad socket or component on the MPU.
Regarding the sound board issue, you should follow the steps below if you've confirmed the voltages are good. (you confirmed this in your above post). If you've replaced the suggested components below, I really don't know what to tell you other than to look for a working board on ebay. They aren't that expensive.
Are you certain the MPU is pushing the correct sound signals out to the board? It honestly still sounds like an MPU problem.
4.12.1 AS-2518-32 & AS-2518-50 Sound Board Troubleshooting
12v regulator section:
Check for 67 volts on the positive side of C17. If missing replace CR3. If positive side of C17 is the same as TP4(43v) replace C17 and CR3.
If 12v is too high, check Q1, R22, CR4, and C18.
U1(4049) can fail. Check for about 2.5vdc where R3 and R5 meet. (On my functioning -50 board, I see .8vdc at this point) If voltage incorrect replace U1 which is a 4049 hex inverter. If still incorrect suspect U4 which is a 4526B programmable 4 bit counter. The 4526 can be purchased from vendors such as Mouser or Digi-Key.
Amplification / no sound issues:
First, make certain that the cabinet speaker is present, connected, and functioning. If an audible hum is heard from the speaker, then the amp is working. Next, check both pots. It is best to replace them if they are the originals (typically blue or black thumb wheel pots). Measuring across the two outer legs of the pot is the total resistance (10k). Measuring from the middle leg gives a resistance with regards to how the pot is adjusted (a value from 0 ohms to 10 kohms depending on the pot's position).
To check the amplifier at U9 (LM741), look for about 2vdc at U9 pin 2. (On my functioning -50 board, I see 6vdc at U9 pin 2. Before replacement, 12vdc was seen when it wasn't functioning.) If voltage is missing, replace the amp.