I have a Black Knight wiht the same problem that you have. When I press the test button on the sound board, It had sound but no speech. And It had a long pause between the last sound and the first sound, like the cpu knew that sould be a speech there.
First I suggest you try with IC 3 at the speech board, change this one first is the easy way. But, In my case, I could find that the problem was a bad pia chip in the sound board.