As per the Rules Compendium at p.14, beating a Listen DC by 20 allows you to pinpoint the 5' square that the sound is coming from, a very important function when facing stealthy or invisible opponents.
The Keen-Eared Scout feat, Players Handbook 2 at p. 80, states that featholders can "determine the precise, current position of the creature or object that caused the sound" if they beat the DC by 10 or more. But it also states that featholders gain a +5 on Listen checks "to pinpoint invisible creatures." I'm not sure how to interpret these provisions. Does the +5 to pinpoint "stack" with the reduced threshold to locate position, effectively allowing you to pinpoint the invisible if you beat the Listen DC by 5?
Say you have Alertness and Keen-Eared Scout, and a normal Listen modifier of +5 (counting only your skill and Wisdom modifier plus the Alertness bonus but not any bonuses from Keen-Eared Scout). There is an invisible spellcaster casting a spell with a verbal component within 10 feet of you. That's a base Listen DC of 0. What is the lowest you would have to roll on the d20 to pinpoint which square the caster is in?
Best Answer
You know the position of a visible creature. It really doesn't matter what sound it makes.
If a creature is invisible, you aren't evem aware it is there, unless you make a great spot check and it's within 30'.
If it makes a sound, you get a listen check. On success, you know of it's presence, but can't see or target it. If you beat it by 20, you pinpoint the location (meaning you can target the correct square on the grid with an attack, but the target still has total concealment and can't be targeted by a spell that targets a creature, with the exception of Effect: Ray).
This is a case where the Rules Compendium does not bring in a rule from an expansion book to the skill description, rather simply from the special abilities description for invisibility.
The PH2 Feat just makes this check better; the listener need only beat the DC by 10 to pinpoint the location, and, in addition adds +5 to these specific Listen checks (versus an invisible opponent). There is no case for where you need to pinpoint the sound of a visible opponent (since you already can see it...). This is a great feat for this specific purpose, increasing the odds by 75% to pinpoint an invisible opponent.
What does pinpoint mean? If you heard an invisible opponent, but you don't know where it is, you can make a melee or ranged attack against it, or attack using a spell that affects an area, but you have to guess the square the target actually occupies, and even if you guess correctly, the attack is against total concealment; 50% miss chance. If you succeed on the Listen check (by either +10 with the feat and a +5 modifier or by +20), you know the correct location (the square on the battle grid) but the opponent still benefits from total concealment and cannot be targeted by spells that target a creature. Note: The KES feat adds the benefit of direction on beating it by 5. That is markedly better than "somewhere" without the feat :)
This was my general understanding long before the Rules Compendium, but I do like that the RC made this explicit (hidden <==> invisible <==> total concealment).
If I am following your example, the DC to hear someone talking at 10' is 0+1, or 1. To pinpoint that person (if they were invisible) would be DC+20 or 21, or DC+10 with the feat or 11. A +10 modifier would negate the roll to hear, know direction or pinpoint.