Sneak Attack is applied when the monster does not aggro. Have noticed I hit with less HP when they hear, see, or detect you in some way.
Move silently and DEX are used towards the move silently roll vs. the monster hearing you.
Hide helps when the monster hears you but cannot see you, like them being blind.
your SA only works in so many hits before the monster notices the big drop in HP and turn to where its coming from