Here's a simple one: Your enemy is 15 feet away and casts Obscuring Mist, then moves a couple of squares so you're not 100% sure which square he's in, but engages one of your allies in melee combat.
QUESTION 1: Can you use the sound of the combat to move up and attack said enemy? The rules simply say you can't use sight-based perception to find him.
QUESTION 2: What kind of move penalty would you impose? Even blinded characters can move at half speed. Do you move any faster because you can see 5 feet?
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THE LONG VERSION:
We were attacked by a group of nightmares, who proceeded to fill our entire area with smoke, limiting vision to 5' and granting 20% concealment in adjacent squares, just like Obscuring Mist. My fighter was 3 squares away from the nearest nightmare, and when I moved him to attack, the GM asked, "How can you get there? You don't know he's there!"
I responded, "Of course I do; he's in the process of chewing on my sorcerer! I can use my hearing to head in that direction, find him, and hit him!"
So we're pretty sure I was within my rights to move 3 squares towards the sound of combat, bringing myself within 5 feet of the nightmare, and then attack, but I thought I'd double-check on the forums. Especially since my searches didn't bring this up in a previous thread. Bizarre!
Well, its tough. Generally speaking the rules here are a bit thin but effectively the creature is invisible to you. To figure out where it is you need to "pinpoint" its location just like you can do with an invisible creature.
So, you need a perception check base DC 20 +20 for trying to pinpoint, -20 for it being in combat, -5 if it moved up to half speed, -10 if it moved more than half, -20 if it ran or charged, +1 for every 10ft away it is.
In your example you could make a DC 16 (20 +20 pinpoint, -20 combat, -5 movement, +1 distance) perception check to pinpoint the location of the creature.
That's pretty much what we ended up doing -- I rolled a 26 Perception, he figured that was ballpark "good enough", and he let me do it. Thanks for the sanity check!