So in the trilogy, we see that when someone has a Hemalurgic spike implanted in them, they can hear Ruin talking to them, both as a vision and in their head. However, we learn in the Hero of Ages that Ruin cannot hear a person's thoughts no matter how much under Ruin's influence they are.
In Alloy of Law, we see that Wax (and other Pathians) uses an earring to "pray" to Harmony, and we see that Harmony can hear his thoughts and respond.
So I guess this leads to three questions: How does Harmony hear the thoughts of Wax, when it's explicitly pointed put that Ruin cannot?
Are the earrings that the Pathians use Hemalurgically charged, as otherwise they would be of no use to Ruin, and therefore Harmony?
Or did Harmony completely change how that aspect of Hemalugy works?
How this all works dates back to the original design of the magic system.
I wanted Ruin and Preservation to be complementary opposites, like many things in the Mistborn world. Allomancy, for example, has Pushes and Pulls were are less "negate one another" opposites, but instead two sides to the same proverbial coin.
Ruin is invasive. The power is more "Yell" than "Listen." The philosopher would probably have some interesting things to say about the masculine symbolism of Hemalurgy and its spikes.
Ruin can insert thoughts. That power, however, can't HEAR the reactions. It's about invasion.
Preservation, however, is the opposite. Preservation listens, Preservation protects. (Perhaps to a fault--if there were no Ruin, there would be no change to the world, and life could not exist.) Because of this, Preservation can hear what is inside people's minds. It cannot, however, INSERT thoughts. (This is important to the plot of Hero of Ages.)
Harmony is both, the two complementary opposites combined. And so, he inserts thoughts with Ruin and still uses Hemalurgy. He can also listen.
Yes, Wax's earring is Invested. (Or, in other terms, it's a Hemalurgic spike.)
Doesn't that imply it was shoved through someone's heart at one point (ala Steel Inquisitor creation process)?
Yes, the metal would have to have been part of a spike that at one point was used to kill someone and rip off a piece of their soul.