Try again with an updated version: http://helicoid.de/scumm/mkspeech.zip
In the end there need to be some more tweaking done like replacing " => ' and ^ => ... (or vice versa) and join multiple lines on an \xFF\xDD\x64\x00.
Ultimately it should directly generate the monster.sou I guess (which shouldn't be that hard to do).
P.S.: As mentioned this is more of a hack than a useful utility. It's just to show how easy you can generate a mapping from the speech.info to the original lines.