The game boots and plays, but all sound effects and music are gone and the speakers give a steady hum.

Constant hum instead of sound: shorted capacitor C26 on the sound board

Cause: Capacitor C26 on the separate sound board had shorted.

Checks

• Swap in a sound board from another System 11 game: if the hum goes away, the fault is on the sound board.
• With the board out, check the capacitors; one that shows continuity is shorted.

Fix

• Replace C26 on the sound board.

Systems

• Williams System 11B

Games

• Taxi

Related tips

• Loud constant hum that the volume control does not change
• How System 11 makes sound
• Which System 11 sound board each game uses
• Using a D-11581 sound board in other System 11 games
• Power-on sound tones and what they mean
• The D-11581 sound board: chips and ROMs
• D-11297, D-11298 and D-12338 sound boards
• Sound circuits on the 11A and 11B CPU boards

Source: Pinball Repair help group!! (Facebook group)