When we pray to God, we can do so in the silence of our hearts, I know. But when we pray to Mary and the saints for intercession, do we have to speak out loud, or can they hear our silent prayers?
You can pray to the saints either out loud or silently; the choice is yours. The reason that they are able to hear our prayers at all is because God grants them that ability. Therefore, if we can pray to God silently, it stands to reason that he has made it possible for the saints to also be able to hear and respond to our silent prayers to them.