The ribbon cable going to the sound board is suspect.
I'd buy a new ribbon cable, but at least reseat your ribbon cable (everywhere it goes) and see if that helps.
Sound board repair is very tricky. I repair a lot of my own pinball boards, but when I send a board to a professional, it's usually a sound board.
If ribbon cables don't fix this problem, strongly consider sending your sound board to a professional, like Chris Hibler.