Concerning the voice of the Father from the clouds in the Transfiguration, Rev. William Smith (PCA) writes:
This is God’s throne chariot. This is the place where he is surrounded by myriads of cherubim and seraphim. Once again we see heaven and earth intersecting. And from this cloud a voice speaks.
The cloud stands at the center of the narrative in Matthew 5. It is the dwelling place of Yahweh. Clouds are judgment (Isaiah 19) or they can be affirmations. Clearly, Yahweh is placing His approval upon the Son. The Son is the chosen One to take on the Mosaic and Elijahic mission to its full completion. The very Son is the One who will soon be murdered, raised, and taken to the clouds to sit at the Right Hand of the Father.