AbdulAlhazred
Legend
I tend to use what Black Night said.. and I tend to be a bastard, when my players attack hide-outs or lairs. Sounds out of the ordinary generate lower DC's for the other Monster's passive perception, etc.
But I'm not really a fan of the idea that a group of monsters 10 squares and a standard wall away, wouldn't notice the sounds of battle between 4 pcs and a guard squad.
Well, it depends on the environment. They MIGHT not. A nice stone wall and some twisty corridors, it might not carry that far. If its an orc lair and the orcs are used to hearing each other fighting they might ignore it.
One thing you have to remember with lairs. A round is 6 seconds long. That really isn't a lot of time. Monsters that are on a hair trigger can certainly react, but most monsters are just going about their ordinary business. Even if they're warlike and always armed, etc they're probably not going to leap up and move to the attack instantly. A whole fight is over in usually under 40 seconds. Its quite easy to imagine even monsters a couple rooms away not responding in that amount of time. Every orc is pulling on his boots and looking for his battleaxe, wondering where the heck the boss is (in the guardrobe most likely) etc. Figure it takes the average emergency squad a couple minutes to roll on a call, its tough to consistently react much faster than that.