We had a situation where the party was separated, exploring in two different directions and one party was beset by an encounter. We decided the DC for the non-engaged party to hear the battle (via a Listen check) would be:

DC=10 + Number of Board sections between non-engaged party members and closest enemy

The number of enemy miniatures then modifies the Listen check result. We figured the bigger the battle, the louder the noise.

Example: Laocoon, the Barbarian is leading half the party down a hallway that holds a chasm, checking to see if any doors lay around the corner. Two zombies attack the original party. Laocoon is 8 board sections away from the closest zombie. He may make a Listen check at a DC of 18, with a +2 modifier to his roll result to hear the battle and react accordingly. 

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