1st: Unplug and clean any sound board connector when pinball off and wait until it is dry if you used a spray cleaner...sometimes I turn the black RCA on my pinball when i got sound troubles and works fine after that.
2nd check ground wires...if you got sound with noise that probably came from a bad ground connection a
But if you got noise without original pinball sound on the backgrounds you might change capa on the auxiliary power supply board( called A5 at the top centre in the backbox)

Could you take a picture?
The connector is probably these used for plug a printer (on the right in the back box)

Also clean connectors, abrade pin on sound board on ribbon cable too