The orcs in the next room over are gambling and arguing about stuff, so they might not notice the PCs walking about with torches, even in full armor. If you've ever gambled with people ever before, you'll know there's a certain level of loudness that goes with it, especially if you're playing for any sort of money. The orcs might not even notice if they're in the next room over, knocking over barrels and crates and ransacking the place. They'll definately notice if one of the PCs is knocking stuff over, or if they start arguing about stuff themselves, so out come the orcs from the next room.
The sounds of combat are even louder, though, so out comes the orcs from the next room down and, in a turn or two, the Ogre five or so feet down. At this point, the entire dungeon is full of screaming, grunting fighters, so just about everybody in the dungeon's about to come down and have some fun.
I don't know if that's the way you're supposed to make it work, but it's the way I like. It makes internal sense (keep it down or the whole dungeon's coming down.), and it splits the dungeon level into three basic parts: Infiltration, Fighting, and Exploration.
|"Creature" by Nicole Cardiff|
After they either get caught or ambush somebody, generally the whole dungeon will start finding its way to the players. This can either result in the players having a fun hide-and-seek, cat and mouse style sneakabout, or it means that waves of monsters are pouring in from every angle until the players either kill them all, run away, or die in the attempt.
After that, they're free to look about and see what the nasties were hiding. This is also when the stationary or sneaky monsters get to play- the PCs are already hurt and low on health/supplies/ammo/whatever, but that's when Goblins and Slimes and stuff are more interesting. The players are going to want to complete the dungeon as fast as possible, since they can never be sure if there are monsters lurking and sneaking around, and they're not sure when the next level will come up and say hello. This is when they explore the rooms, deal with cartography, treasure, and the like.
This might not be the best way to do it, but it feels natural, somehow, to have it split up like that, and really makes the dungeon feel more alive than if each room is full of orcs who, apparently, can't hear the battlecries and death-screams of their buddies across the corridor.