I thought that the sound pot on the small speech board was more of a mixer pot? Meaning you could have higher music over speech, or vice versa.
I had a comet at one point that had normal music but speech was low. I replaced both op amps and the 55516. After I had replaced the 55516, I had to adjust the pot. Not sure if any of this helps
Here is a link to my old thread:
https://pinside.com/pinball/forum/topic/comet-low-speech-volume