Objection 1. It would seem that one angel does not speak to another. For Gregory says (Moral. xviii) that, in the state of the resurrection "each one's body will not hide his mind from his fellows." Much less, therefore, is one angel's mind hidden from another. But speech manifests to another what lies hidden in the mind. Therefore it is not necessary that one angel should speak to another.
Objection 2. Further, speech is twofold; interior, whereby one speaks to oneself; and exterior, whereby one speaks to another. But exterior speech takes place by some sensible sign, as by voice, or gesture, or some bodily member, as the tongue, or the fingers, and this cannot apply to the angels. Therefore one angel does not speak to another.
Objection 3. Further, the speaker incites the hearer to listen to what he says. But it does not appear that one angel incites another to listen; for this happens among us by some sensible sign. Therefore one angel does not speak to another.
On the contrary, The Apostle says (1 Corinthians 13:1): "If I speak with the tongues of men and of angels."
I answer that, The angels speak in a certain way. But, as Gregory says (Moral. ii): "It is fitting that our mind, rising above the properties of bodily speech, should be lifted to the sublime and unknown methods of interior speech."
To understand how one angel speaks to another, we must consider that, as we explained above (Question 82, Article 4), when treating of the actions and powers of the soul, the will moves the intellect to its operation. Now an intelligible object is present to the intellect in three ways; first, habitually, or in the memory, as Augustine says (De Trin. xiv, 6,7); secondly, as actually considered or conceived; thirdly, as related to something else. And it is clear that the intelligible object passes from the first to the second stage by the command of the will, and hence in the definition of habit these words occur, "which anyone uses when he wills." So likewise the intelligible object passes from the second to the third stage by the will; for by the will the concept of the mind is ordered to something else, as, for instance, either to the performing of an action, or to being made known to another. Now when the mind turns itself to the actual consideration of any habitual knowledge, then a person speaks to himself; for the concept of the mind is called "the interior word." And by the fact that the concept of the angelic mind is ordered to be made known to another by the will of the angel himself, the concept of one angel is made known to another; and in this way one angel speaks to another; for to speak to another only means to make known the mental concept to another.
Reply to Objection 1. Our mental concept is hidden by a twofold obstacle. The first is in the will, which can retain the mental concept within, or can direct it externally. In this way God alone can see the mind of another, according to 1 Corinthians 2:11: "What man knoweth the things of a man, but the spirit of a man that is in him?" The other obstacle whereby the mental concept is excluded from another one's knowledge, comes from the body; and so it happens that even when the will directs the concept of the mind to make itself known, it is not at once make known to another; but some sensible sign must be used. Gregory alludes to this fact when he says (Moral. ii): "To other eyes we seem to stand aloof as it were behind the wall of the body; and when we wish to make ourselves known, we go out as it were by the door of the tongue to show what we really are." But an angel is under no such obstacle, and so he can make his concept known to another at once.
Reply to Objection 2. External speech, made by the voice, is a necessity for us on account of the obstacle of the body. Hence it does not befit an angel; but only interior speech belongs to him, and this includes not only the interior speech by mental concept, but also its being ordered to another's knowledge by the will. So the tongue of an angel is called metaphorically the angel's power, whereby he manifests his mental concept.
Reply to Objection 3. There is no need to draw the attention of the good angels, inasmuch as they always see each other in the Word; for as one ever sees the other, so he ever sees what is ordered to himself. But because by their very nature they can speak to each other, and even now the bad angels speak to each other, we must say that the intellect is moved by the intelligible object just as sense is affected by the sensible object. Therefore, as sense is aroused by the sensible object, so the mind of an angel can be aroused to attention by some intelligible power.
Source: Summa Theologica (New Advent)