"Then a voice came out of the cloud, that said, "This is my Son, My Chosen One, listen to Him!" And with the dying away of the voice Jesus was found alone. At that time they kept still and told no one what they had seen" ~ Luke 9:35-36, Berkeley
God caused a cloud to overshadow the eager disciples. Peter, particularly, needed to be stopped in his impulsive plans. Gently, but firmly, God stopped him with a cloud. "Then a voice came out of the cloud..." God never stops our headstrong actions without offering an alternative. When we hear none, or see none, it is because we have been made deaf from the explosion of our egos.
"My soul, wait thou only upon God ..." When we do wait upon God, the voice comes out of the cloud around us. That day on the mountain, the voice said: "This is my Son, My Chosen One, listen to Him!" That was all. That was enough. God had spoken the one thing which covers all confusion. He directed their attention back to Jesus.
"And when they had lifted up their eyes, they saw no man, save Jesus only." This is what God intended. Everything He has to say to us is in Jesus, His Son. Apparently, after they looked back at "Jesus only" once more, they must have known. The account says "they kept still and told no one." They evidently had something to tell. They had looked at Jesus only.
"... behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him."