The sound board is completely silent.

No sound at all: check the -5 V regulator

Cause: The 7905C regulator VR1, which makes -5 V from the -12 V input, has failed and let -12 V through, destroying the chips on the -5 V line: LM833s at U28 and U30 (Batman to Batman Forever), or the 4052 at 5B and MC3403s at 3B, 6B and 7B (Laser War to Ninja Turtles).

Checks

• Measure -5 V across C49, red probe on the leg that joins VR1, black on the ground leg.

Fix

• Replace VR1 and every chip on the -5 V line.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• No sound, or weak sound
• No sound after the 12 V plug went on the speaker lead
• Check the sound board's regulated -5 V and +5 V
• Sound board versions are not interchangeable
• Parts on the A5 auxiliary supply
• The 520-5050-00 sound board variant
• Data East and Sega sound boards by game
• No sound with all voltages present: the PALs

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)