Sound boards can be black boxes since they aren't documented nearly as well as the rest of the boardset.

You'll have to use the schematics to figure out what the edge connector pins are for so you can verify the voltages. It's also a good idea to check the diodes, make sure the nuts for the regulator are tight, and clean off the edge connector contacts with a pink rubber eraser.

http://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_1#Sound

http://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_1#Multi-Mode_Sound_Board

http://www.pinrepair.com/sys1/#sound

Unfortunately, this particular board has a fatal flaw in which the two unobtainable chips are known to go bad, so like the MPU, it tends to be a throwaway board more often than not.

There is a MIOT adapter board you can use in place of the bad chips: https://coinop.mally.eu/produkt.php?bestellnr=101647&details=1

Or, you can replace the whole thing with an aftermarket board:
https://www.flippp.fr/pifx1.php (from France directly, or US suppliers like bigdaddy-enterprises.com sometimes carry them)