Listening through a Door

Perception DC*	Sounds the PCs Hear
Monsters' Stealth check + 5	Monsters moving quietly around the room
5	Battle in progress, or agitated or dramatic conversation**
15	Normal conversation or ritual casting**
15	Doors opening or closing (and similar sounds)
25	Quiet conversation, whispers*
35	Battle preparations (weapons being drawn, and so on)

* Add 2 to the DC if the characters are more than 10 squares away.

** If you succeed by 5 or more, and you know the language being spoken, you can understand what the creatures are saying.

Published in Dungeon Master's Guide, page(s) 37.