All sounds are fine but speech is barely audible, and the speech board pot only changes the effects volume.

Speech very faint on a type 2 board

Cause: A bad 1458 at U3 on the sound board (reported on Blackout).

Checks

• Confirm sounds and speech are all present but speech is weak.

Fix

• Replace the 1458.

Systems

• Williams System 6 / 6A
• Williams System 7

Related tips

• Lamp types found in EM games
• Speech drops out after 20-30 minutes, back after a reboot: speech chip on the MPU
• Sound effects work but speech is missing or very quiet: replace the speech board op amps
• No speech? Check the speech board pot
• Type 1 and type 2 sound boards and their switches
• Setting up a sound board for speech
• Using a 6802 on a type 2 sound board
• System 6/7 sound and speech board ROMs

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)