As the title suggests... why are people who are invisible, also apparently silenced, even though they're running all about the place, sometimes in full plate and the like, yet nobody is able to hear them, or detect them, as they disturb their surroundings.
I feel like having a static X DC for listen to hear someone running while invisible about the place would be a good idea, with a higher DC, for those who take the time to walk or sneak about covered up.
Just because someone can't see you, doesn't mean they can't hear you.