Not just a separate space but an entirely different dimension from us, if one can speak of eternity as a dimension. In any case speaking and hearing are, here on earth, functions of vibrations in the atmosphere and one presumes that angels, spirits of every kind, and the dear departed do not have an earthly environment in which to hear vibrations in the air. So, the saints "hear" by some other means, perhaps the same or similar means through which God "hears" our prayers. And since we do not live in eternity and have not experienced it ourselves we are not in a good position to comment on what is or is not possible there.
In the case of Samuel and Saul with the witch of Endor somehow or other Samuel "heard" the call to come and converse with Saul. He did come, shocked the living daylights out of the witch (who was no doubt a charlatan much like mediums today are) and let Saul know his fate was sealed and he would soon be dead. This is one example of the dead "hearing". Another is at the Transfiguration of our Lord, where Moses "heard" Jesus, and the three apostles "heard" Moses, how it works I do not know, but work it does.
Another example is the souls of the dead under the altar in heaven who "hear" of the state of the world and its injustice so that they cry out to the Lord, "how long" until they are avenged.
Three examples of this kind of "hearing" ought to be enough, even if we do not know how it works.