This may or may not be helpful, but I recently had to rebuild both a sounds plus board and a vocalizer board for similar issues on my project xenon. I was missing a few pieces of speech from the vocalizer board (hissing instead of speech), sound was all over the place and it would randomly just not work at all.
In the end I replaced the following:
- all caps
- board headers
- replaced the IDC connector with a molex
- new sound and speech pots
- all sockets replaced
- new roms as old ones were corroded
- replaced CR1 - CR3 on the vocalizer board with 1n5404s mounted further off the board to help keep them cooler.

I also had to switch the jumpers on the vocalizer as well to support the new roms. Vid1900 covered the vocalizer jumpers in a topic he posted a while back here: https://pinside.com/pinball/forum/topic/xenon-vocalizer-eprom-repair-vids-guide.
Technically, I probably just needed to replace the caps and roms, but I wanted the board to be updated so I wouldn't have to touch it again.

Mine works great now. I still need to replace the volume pot on the coin door, but mine appears to be working fairly well thus far.