Press the F3 key a few times until the speech doesn't get cut off anymore. You can use the intro cutscene to test that. If the text ends up lingering too much after the speech line ends, you can press F1 once, to make the text a bit quicker again.
You can read all the hotkeys info in the ScummVM user manual
Take into account that the speech and text not always coincide. Some of the lines were re-written for the voice actors probably. I recommend choosing one or the other using the T hotkey to test them and then edit the game's INI file to set it permanently. If you activate speech only, there are no interruptions.
You can set speech only or text only with the options of the INI file for the game, but not the text speed as far as I know ("talkspeed" in the INI file does nothing for these games). If you use text only, and it goes by too fast, you will have to use the F3 key every time the game starts.
The default lines (for the speech and text) are:
To set the game for speech only edit them to:
And to set it to text only: