Do we have to speak out loud for the angels to hear us when we pray to them or speak to them?

First, we must be aware of the fact that the angels cannot, as mentioned in the last issue of our Circular Letter, read our minds. Nevertheless, they can know specific thoughts that we willfully desire to communicate with them. The essence of all spiritual communication is the decision that we make to transmit a thought or an idea to another person, be it an angel, a devil, a saint in heaven, or God Himself. Therefore, as soon as we make the decision to speak with or to pray to the angels, they will become aware of what we want to communicate to them.

St. Thomas Aquinas taught that even "when our Guardian Angel is in Heaven he knows what is happening to man" (cf. Summa theologiae, q. 113, a., 6, ad 3). Our Guardian Angel can learn about what we want to communicate to him from the Beatific Vision of God, should God will to communicate this to him. Dr. Peter Kreeft observes that, "God tells the angels all they need to know about what is going on in our world. They don't need to learn about it through sense experience (sic). [In other words,] … angels see the world reflected in God's mind, somewhat as a listener would see the events of a story told to him by its author" (Dr. Peter Kreeft, Angels and Demons, question no. 37, p. 67). However, this opinion, if taken too far, would remove the need for any communication, not only between angels and men on earth, but also among the angels and saints in heaven. Yet, the moral good of the communion of saints also entails the social dimension of interpersonal relationships which are expressed by acts of communication of information and acts of love. Moreover, this position ignores the common doctrine regarding the Heavenly Hierarchy of the angels. In this regard, St. Thomas and theologians in general hold that God reveals His plans to the Highest Choirs of angels, who then are His ministers in communicating this knowledge and directing the ministries of the lower angels. From this it would not follow, that each angel, in virtue of his own beatific vision, receives all the knowledge that is pertinent to his ministry. The basic principle behind the angelic hierarchy is this: God communicates to His creatures a double share in His goodness: a) the goodness of being (the natural perfection of their nature) together with grace; and b) the goodness whereby they are collaborating causes in the economy of creation in that they causally contribute to the growth of lower creations in perfection.

Finally, while it is certainly within the power of God to reveal the secrets of the heart to any angel, there is no cogent reason why this should generally take place, since it would mitigate against the perfection of the social order in the communion of saints. While the angels may well know our spiritual needs even without such a revelation, our prayers to them are not without purpose. We mention here just two. First, the merit of prayer and the power of hope are necessary conditions for the acquisition of many graces. Hence, the mere knowledge of our need does not entitle, in itself, the angels to mediate to us every grace. Secondly, in and through the communication of prayer we also concomitantly manifest to the angels the dispositions of our heart and soul, which knowledge, in good part at least, pertains to the secrets of heart. This knowledge of our interior dispositions is of great pastoral value to the angel, since by it he can better attune his ministry to our inclination and capacity to cooperate with grace - http://www.opusangelorum.org/membership/questions_answers.html.