Well...
I sorta found out what was wrong...
I was using the floppy version of the files with the soundfile from the cd version, so the game didn't even look for the specific sounds when characters spoke.

Hello! Great job in ScummVM, everyone! It's simply great! I have only tried playing DOTT so far, but I have a slight problem. I have installed it correctly (with MONSTER.SOU). I get all the sound effects (like the bird choking in the intro, or the tentacles "jumping" around etc., but I get...