Go to 'Settings' then choose 'User-defined variables' then find all the fields that set the enemy's sounds. There is a field for activation, idle, notice, attack, etc. Scroll through and check carefully, because I'm sure I missed a few!
Silent monster eh? Nasty. Don't use that to sneak up on innocent folks.....
By the way, that "screeching, high-pitched" sound you're speaking of is part of the game engine. It always plays when any monster (with the exception of the water lurkers, if I'm not mistaken) is chasing you. Basically, it means RUUUUUUN