I don't know if this is pertinent as your game is made by Bally, but on my Williams Black Knight, I had a similar problem, where the pot attached to the sound board was a way to balance the level of the sound effects vs the speech, and the pot under the playfield, controlled the overall amplification.
It started out with sound effects playing clear and loud, with no speech. If I turned the playfield pot up as high as it went, the sound effects would be deafening, but the speech could be barely heard. If I turned the pot under the playfield down so the sound effects were at the correct level, there was no speech at all.
I realized that the pot on the sound board was more of a mixer. One extreme had full sounds and quiet speech, the other extreme had full speech and quiet sounds. I ended up finding a happy medium with both effects and speech at an acceptable level, that took some tweaking of the pot on the sound board. After doing that I was then able to adjust the overall volume with the pot under the playfield.
Hope this helps, it took me way too long to realize the sound board pot wasn't a low-high analog volume control but more of a left/center/right kind of adjustment knob.