Also with noting: i recently discovered that the flippers have an additional switch mounted to the end of stroke switch--that extra switch will have gold contacts. Those extra switches are what activate the sounds when the flippers are flipped.

I would definitely re-cap the sound board as the above poster suggested, and also re-flow solder to the header pins and replace the female connector(s).

Also check the voltage coming into the sound board, it could potentially be a power supply issue as well.

Just to add to that suggestion: the switch with gold contacts (the switch that activates the sound when flippers are flipped) should be cleaned with a business card or clean piece of paper--you don't want to file gold-contact switches. The high-current end of stroke switches can be filed, however.