I just fixed a system 6 board last week - it drove me nuts for ages. this board had a similar problem. Some sounds (but not all) in solenoid test & in game play. All sounds/speech correct in sound board self test.

After grounding the inputs on the sound board all but one worked, the one that didn't work also derailed all other sounds/speech to silent after that until switching machine off/on again to start again.

As it looked like an input problem on one line I assumed it would be either the buffer IC5 (4050) or IC6 (4068) or the PIA. I replaced all & no change. I was going to get some new caps later, but I didn't think that was the problem for this.

I tried one last thing, determining if it could be the RAM IC11 (6810) even though the 6810 didn't seem to be related to the input line. To eliminate the 6810 as the fault cause, rather than desoldering it, I swapped out the 6808 for a 6802 & simply cut trace W14 (as per jumpering instructions on page below) so the 6802s internal 6810 RAM is now used & not the boards 6810. very easy to do.

http://www.firepowerpinball.com/downloads/newlogic/18-soundlogic.pdf

Now I had sounds/speech from all inputs, the 6810 was the problem, still not working 100% though, still some strange incorrect sounds occasionally, but close to perfect. I decided to remove the 6810 from the board altogether, this now fixed the board, 100% working, furthermore, even leaving the chip in the circuit while not being active still caused some problems.

You could try this, see if this fixes it or if their is any change, easy to do if you can get hold of a 6802 & easily reversible if you want to go back to a 6808. Also, if you have a AMI brand 6810 RAM (mine was), even more chance it's faulty as this manufacturer is known for having a high failure rate.

Dips should be this way too.
IMG_9231 (resized).JPG

W14 trace cut with a sharp stanley knife to enable the 6802.
IMG_9232 (resized).JPG