The Earplugs are one of the items that are necessary to solve the riddles surrounding the acquisition of the Key of Tears from inside Raysiel's Tower. Their location is inside the Raquia Maze. They bear the symbol of Raquia.
We have to wear them when entering the Siren Room, otherwise we'll be damaged by the humming sound originating from the floating Siren figure. Our prudence will reward us with a nicely worked Silver Key, used for accessing the small Chapel in Raysiel's Tower. Nota bene: if we approach the figure without the earplugs, our movement will be slowed down and we won't be able to pick up the key.
The Earplugs and the Siren are an allusion to the myth of Odysseus or Ulysses, respectively: lest they be driven insane (and shipwrecked in the process) by the enchanting song of the Sirens, Odysseus ordered his oarsmen to occlude their ears with beeswax. When inspecting the floating siren figure, Rebecca will notice a certain similarity to a picture she once came across in a book about Odysseus.
Adam: Earplugs. Why am I suddenly thinking of Odysseus?