As we all know as of the end of volume 3 Pyrrha was killed by Cinder. It seems her soul had scattered to the wind but what if I was to tell you that Ruby's silver eye power absorbed Pyrrha's soul?
This theory begins to take shape in episode 2 when Ruby begins to have nightmares with Pyrrha's voice calling out to Jaune. Ruby wakes to find Jaune in the forest practicing a good ways away from the campsite making it extremely unlikely she heard the recording of Pyrrha that he is using to practice as well as Ruby only hears Jaune's name repeated over and over no real pause.
It is not humanly possible for her to have heard it. This makes me think that since Ruby is with the remainder of JNPR she is prone to hearing Pyrrha call out to her leader as she is sleeping across from him.
I believe when Ruby witnessed Pyrrha's death and when the soul scattered her power triggered absorbing Pyrrha's soul allowing her to hear Pyrrha calling out to Jaune.
As of now we know nothing more but I will touch up on this on if information becomes available.