The transfiguration was a moment of surprise and very special for the disciples at that time, since the glory of God was shown through a cloud. But something that powerfully caught our attention was that magnificent voice that spoke to them at that moment.
But it is good that we read Matthew chapter 17 in its verses 5 to 8, here we can see what happened clearly that day of the transfiguration of Jesus:
5 While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
6 And when the disciples heard it, they fell on their face, and were sore afraid.
7 And Jesus came and touched them, and said, Arise, and be not afraid.
8 And when they had lifted up their eyes, they saw no man, save Jesus only.
Matthew 17:5-8 (KJV)