Seems I have very similar or the same problem. https://pinside.com/pinball/forum/topic/williams-speech-board-extremely-low-volume

FYI, schematics can be found here: http://blackknightpinball.co.uk/wp/circuit-board-diagrams/speech-modules
There's 2 different versions, check what chip is in U1.

Edit: found this from ThisOldPinball/Marvin3m archive:

Faint Speech from Type2 Sound Boards.
A reader reported, "While testing the sound/speech board for my Blackout, I hear all the sounds perfectly. Also all the speech is there, but is very faint, almost impossible to hear. Turning the pot on the speech board only varies the volume of the sound effects, not speech volume. Is there some sort of mixer on the speech board that could be bad?" The answer was a bad 1458 sound amplifier at U3 on the sound board.

But I don't see a 1458 on the sound portion of the sys9 driver board, just the two on the speech board.

R14 and R16 are both 10k, but will measure incorrectly in circuit because they connect back to each other through the pot. If they measure near 6k on the board, they are likely fine. Only way to know for sure is to unsolder a leg.

R15 is 27k, and should measure correctly on the board.

Replacing U3 fixed mine.