Help with enemy noises

[Mackiiboy]

1. Open a monster entity you want to edit
2. Press Settings -> user defined variables
3. Scroll down till' you find AmbientSound_Idle, AmbientSound_Alert, AmbientSound_Hunt
4. Remove the sounds so nothing is written, leave them blank
5. Save it as a new entity

About the mask over a monster's face, you could attach a billboard to one of the bones in the monster. The billboard-type should be FixedAxis.

What would I do in the model editor?

Also how do I place a mask or something over the monster's face? Sort of like that one custom map where someone put a nyan cat face on a grunt as well as a troll face. Howtododat?