Flash background sound missing with green ROMs
On Flash with green flipper ROMs and the green game ROM, set adjustment 33 to 01 to get the background sound. If that does not help, also try adjustment 31 set to 01. The yellow ROM version does not seem to have this problem.
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Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)