there's a green wire harness that goes between the rom and sound boards. this hardware used the pinball method of calling sounds, where only a single sound plays at once. there's 6 data wires, grounding each should play the according sound. after that, it's combinations of wires to play sounds. if any of those wires aren't firing, you'll lose the primary sound and any combination that requires that signal.
go into test mode, then sound test. let it cycle through sounds. if any don't play anything within the first 6, you're either looking at bad solder on the rom or sound board ends, or one of the wires pulled out of the harness. reflowing the header pins and crimping new molex ends will definitely fix this.