Have you noticed that in DD some objects give you a 'Hearing' boost?
Im looking into the manual, but I don't see any explanation about it. Im sure its an obvious thing, but Im intrigued.
Maybe it's a passive ability to detect invisible enemies?

Magical items provide bonuses to stats, to resistances, to sight and hearing, to skills, and charm slots.
Better hearing and sight stats allow you to hear / see enemies from far off . . .meaning they will show up on your mini-map as red dots much sooner than they usually do.
Cause . . . you can hear and see better . . . =)

Yes, yes, I noticed the "sight" state, since Im improving it by "elven sight" with the skills.
But hearing is somewhat intriguing since it only appears as bonus on some equipment. If sight makes you see NPCs from afar, what's the difference between sight and hearing?

I don't know that there is any REAL difference between the improved sight or hearing. Mostly either will make an enemy show up sooner as a red dot on your mini-map. Possibly having both may even make them show up even sooner.
The implication is that you "hear" the critter from further away and therefore are aware of it more quickly due to the enhanced hearing. Someone else may be able to give you specifics about the differences between the two if there are any . . . =)

Can't say it applies specifically to DD, but generally the hearing stat is used to try and detect beasties\thingies that you can't see under normal circumstances - eg, behind a closed door, around a corner or in the darkness beyond your source of illumination.

Yep. the common sense works here ^^ (thanks both)
It seems that the hearing stat lets you mark NPCs in the minimap on places not directly under your vision (like corners and inside houses)
Im really impressed with DD's deep gameplay :O