Take a close look at your option switches on your sound board:

https://www.pinwiki.com/wiki/images/d/d5/Wms-sys6-7-soundspeech-assembly.pdf

Up top, next to 10J3. See how they are set. Might be a bad connection or bad switches.

Well, according to the chart, ROM 5T 4971 contains those four words. That would be IC7 on the speech board, so it might be worth looking at the socket and the chip.

I also think (at least looking at a thread about Gorgar) that you can swap roms around to test them. Might be worth a shot to see if you don't get sound from a socket or sound from a ROM.

If you're careful you can get those chips out with a flathead screwdriver, just slowly. It's also worth pulling the board out to look at the solder connections.

I mean there's also the option of buying a replacement modern board if you're worried about the work, but this also good troubleshooting.

I'm sure someone smarter than me will chime in soon, or perhaps ask in the FirePower thread, too.

Maybe try swapping next, see what happens.