A invisible caster with no move silent skill is a dc 10 to hear.
2 guards whispering at 100ft distance is normally a dc 35 to understand.
However, can the sound of battle increase this DC? Does the sound of crowded market increase this DC?
I know there is a +5 for listener distracted, but when is this taken into account? Do you get this -5 each time you hear something in a reactive manner, do you get a -5 for every action you do in your turn (for example, balance check + full round attack + attack of opportunity = 3x distracted = -15)?