Emu's put together a couple of example clips that show how new speech can be inserted. Obviously this isn't just a recording placed on top of the video - he's managed to trigger the sound file to natively play. If you'd like all the wonderful technical details explaining how this is possible, check out his full explanation here. It would be quite an undertaking, but this demonstrates how it would be possible to do a full audio mod for the game!
I have found a way to add speech-play operations to the game script. At present, this is modified by hand, so I only added a few lines (and the speech are found in other plots, please ignore the content). [FM Towns speech depicted] What is more, even in the FM TOWNS version, there is no speech in this plot. I use Shadow's communication speech here (in a new file named SPEECH.S30), plus Blair's opening speech.
You could hear Thrakhath's voice "Yes, my Emperor." at the beginning, where there was no voice and even no text in the original script. Thanks to UnnamedCharacter! The fantastic research makes this possible.