The testimony of the Fathers

"O God, who in the glorious Transfiguration of your Only Begotten Son confirmed the mysteries of faith by the witness of the Fathers and wonderfully prefigured our full adoption to sonship, grant, we pray, to your servants, that, listening to the voice of your beloved Son, we may merit to become co-heirs with him. Who lives and reigns with you in the unity of the Holy Spirit, God, for ever and ever."

"Deus, qui fidei sacramenta in Unigeniti tui gloriosa Transfiguratione patrum testimonio roborasti, et adoptionem filiorum perfectam mirabiliter praesignasti, concede nobis famulis tuis, ut, ipsius dilecti Filii tui vocem audientes, eiusdem coheredes effici mereamur."

     Collect for the Feast of the Transfiguration, current Roman Missal and translation.  Tridentine missal, as trans. Officium divinum:

"O God, Who in the glorious Transfiguration of Your only-begotten Son strengthened the mysteries of faith by the testimony of the fathers, and, by the voice coming down in a shining cloud, miraculously betokened the complete adoption of Your children, mercifully grant that we be made co-heirs with that King of glory, and sharers in that same glory.  Through."

"Deus, qui fidei sacramenta in Unigeniti tui gloriosa Transfiguratione, patrum testimonio roborasti, et adoptionem filiorum perfectam, voce delapsa in nube lucida, mirabiliter praesignasti, concede propitius; ut ipsius Regis gloriae nos coherendes efficias, et ejusdem gloriae tribuas esse consortes.  Per."

The missal in Latin and English, being the text of the Missale Romanum with English rubrics and a new translation (New York:  Sheed & Ward, 1949), 1118:

"O God, who in the glorious transfiguration of thy only-begotten Son didst confirm, by the witness of the prophets, the truths revealed to faith, and by the voice speaking out of a bright cloud didst miraculously signify the fulfillment of our adoption as thy children; in thy mercy deign to make us co-heirs of his kingdom and sharers in his glory; through."

Etc.  E.g.:

"O GOD, who in the glorious Transfiguration of Thine only-begotten Son didst confirm the mysteries of faith by the witness of the Fathers, and in wondrous wise didst fore-token the perfect adoption of sons by the voice descending from the shining cloud; mercifully grant unto us to be made coheirs with the very King of glory and bestow upon us a partaking of His glory. Through."

     Though there is something to be said for both versions, and though I often prefer the pre-Vatican II one, I rather like this shift from the voice of the Father to the voice of the Son, i.e. the substitution of ipsius dilecti Filii tui vocem audientes ("listening to the voice of your beloved Son") for voce delapsa in nube lucida (by the voice carried down in the shining cloud), and the way in which the phrase is repositioned.  Both collects speak of a prefiguration of our adoption as sons, but the contemporary one makes what the voice expects of us a condition of our perfect final participation.
     This would explain why I'm not finding transfigurat* (and therefore the collect?) in the ancient sacramentaries:  "The Feast of the Transfiguration, observed on 6 Aug., originated in the E. Church, originally local, but widely adopted before ad 1000. In the W., where the feast was not introduced till a much later date, its general observance goes back to 1457, when Callistus III ordered its universal celebration in commemoration of the victory gained over the Turks at Belgrade on 22 July 1456, news of which reached Rome on 6 Aug." (Brandon Gallaher in ODCC4).