[3.5] Mongrelfolk, modified (PEACH)

[Debihuman]

It's a DC 10 Listen check to understand a conversation taking place less than 10 feet from you.
Using a Listen check for this makes sense to me.

Normal conversation, yes. I agree

Also, Extraordinary abilities don't defy the laws of physics, they're completely mundane abilities.
They just have different physics.

Actually, on this you are incorrect. Here is the full definition of an Extraordinary Ability (I did quote from it earlier):

Extraordinary Abilities (Ex)

Extraordinary abilities are nonmagical, though they may break the laws of physics. They are not something that just anyone can do or even learn to do without extensive training.

These abilities cannot be disrupted in combat, as spells can, and they generally do not provoke attacks of opportunity. Effects or areas that negate or disrupt magic have no effect on extraordinary abilities. They are not subject to dispelling, and they function normally in an antimagic field.

Using an extraordinary ability is usually not an action because most extraordinary abilities automatically happen in a reactive fashion. Those extraordinary abilities that are actions are standard actions unless otherwise noted.

Hence, they may break the laws of physics as I wrote earlier. It is not a DC 10 listen check as per normal conversation, this is an Extraordinary Ability, which should increase the DC greatly. That's why these are saves and not checks. So yes, the -8 circumstance penalty should apply if you make an Listen check to hear a mongrelfolk rather than another humanoid because the mongrelfolk have this Special Ability. A simple Listen check at DC 10 does not cut it.

Here's a nicer way of putting it: At DC 10 you can hear the mongrelfolk. If you make your check by +8 or more, you uncover the ruse.

Debby