You put the files with your resource (best choice), or wherever next to your model folder.
Open ModelEditor and press File->Open, Select a monster o3d file (example, Mvr_Aibatt.o3d).
It will load the monster with animations, when you select an animation on the right side, it will show some dots on the bottom panel..
Double click on the colored dots to remove the sounds, and to add a sound, double click wherever you want the sound to start.
This window will open:
Then you choose the sound you want the monster to make.
SND_* ID's can be edited with definesound.h & sound.inc
DefineSound.h is used to add a new ID
Sound.inc is used to add a sound to that ID.
So in short,
Select animation, select what sound to play on that animation.
When done, Go to File-> Save
You can also check here on what gives what.
Hit/shoot -> Plays correct SFX if added (Cannot be chosen in modeleditor)
Sound -> Plays sound
Quake -> Makes the world ingame Quake, is used mostly for bigger boss monster such as Clockworks, Behe etc..