Well, it is just providing you much more control of the resources (perhaps more control than is required in some areas, given other limitations). The model conversions done are actually just specifying information about that "entity" in game, such as physical properties, attaching sounds and telling the game if that entity is an enemy, or some other form of physics object. Sadly, the documentation on the model-editor is sparse (I've spent hours trying to work out what a
certain check-box does to no avail).
The .snt file is again like this. For straight-forward sounds activated via script and other means this isn't required, but for enemies the .snt file provides information about the sound, for example it allows you to pick one out of a set of sound files to play at random, and also specifies volume information and details about blocking IIRC.
These are both just text xml files, however you get a model-editor for the entities as there are quite a lot of options there.
As for the voices, the hardest part is putting in the lines and stuff in the language file and setting up the language and flashback files. I haven't really played around with this myself as I haven't really had to do subs.
Edit:
Just took a look, open up the flashbacks folder and you will see some .flash files. Open in note-pad, and these are again some xml files:
<Voice VoiceSound="flashbacks/CH01L03_MapRoom_01.ogg" EffectSound="" TextCat="Flashbacks" TextEntry="CH01L03_MapRoom_01" />
Is a sample line. So it looks like you specify a sound file as the bit, then the catagory and entry in the language file which corresponds to that line.