sorry, forgot to post my results, thx for reminding me andré

- first thing i did was exchange the two flat cables (sound board and voice board) with my new scsi cables - no change, no speech
- exchanged the voice generators (the big chip) of the two boards - no change, no speech
- exchanged the voice roms (two chips) of the two boards - no change, no speech
- tried the chips in various combinations - no change, no speech
- switched the scsi cables for different ribbon cables - no change, no speech
result of my tests: NO CHANGE, NO SPEECH
one thing i remember from when the speech failed during gameplay: the sound of the laser fires got definitely louder. at one point when i was doing some testing today, the laser fires suddenly weren't that loud anymore, as if the speech was trying to "break through", but then the sounds got louder again ...
fact is: both boards are definitely NOT working - sorry michael

! andré, you said that voice generator chip can be reordered, but it isn't cheap, right?
any other suggestions?