It doesn't matter where it goes so long as it is connected to the ground braid in some way.
I keep saying to attach the ground wire and MartyJ thought that it was a "Ground issue or MPU board issue".
So please fix the ground problem and lets see if it fixes your sound problem. If it doesn't fix you particular sound problem, we are at least one step closer and your pinball will be correctly wired.
+1.
Door must be grounded and it has popped off or fallen off. Make sure to clean the paint away (enough for screw head) so it makes good ground. It would have been connected previously so it should be easy to see.
This will eliminate one problem as Humpalot suggested before moving on to next.
Have you a Digital Multimeter at home? You won't need it for the next step, but if you do have it handy.
