In test mode under MAME (I don't have this game), you can do a sound test with the joystick. Looks like that's all this test mode does, as far as interactivity. If yours won't do that, then it sounds like the CPU is crashed (which could have a number of causes).
If I understand what you said, it worked fully when someone else tested it outside the cabinet. So either the board/component was damaged/zapped (or socketed part came loose) when it was being handled, or the problem is in the cabinet (power supply, or wiring harness).
If there's a test mode button in the cabinet, check that it's not stuck or shorted in the wiring.
Does the wiring harness connect to the board through an "FCC filter" (aka RF filter) board? I've seen people mention those being problematic, it's just another possible failure point. You could try bypassing that.
BTW Hi DreamTR, thanks for sharing your NES stuff, years ago I've seen some of the fascinating carts you had loaned to kevtris.