Thjng with pinball is you have to do a bunch of things to find what isn't working.... Now that we know it's grounded correctly and the sound works again (from the help above) we can move on to the next bit of troubleshooting.
In test mode do the coils fire as they should?
Take a look at the Driver board. Do any of the transitors (little black squares with 3 legs) look burnt or broken? Could be that.
Check the edge connector at J6. Do those look ok? Making strong contact with the board?
I'm currently having an similar issue because I had to snap some MPU chips for another project which had bad IC chips.
It could also be the long black computer chips in your MPU board. Z13, Z15 etc. 7400 IC chips. They could be dead. They control switches, switches often tell coils to fire. Some switches work but some don't? If the hame doesn't push the ball into the shooter lane the game doesn't likely know to start the sound...