Also check that the sound board ROM is correct for Firepower, Sound 3.

If the problem were in speech amp circuits, then your board would have silent pauses when it tries to say something during game or sound test. Since you don't have those pauses, the sound ROM does not recognize speech board and thinks it is not installed - or misreads the speech enable DIP switch.

Yes, it is supposed to adjust the balance between speech and sounds so it also affects sound volume.

Scanbe sockets are best replaced by first prying off the black plastic frame, then taking the pins out one by one, warming pin with soldering iron and pulling it out with tweezers. After that, use solder wick or desoldering pump to clear holes. Don't try to desolder and remove the whole socket at once.

Good catch!