What you understand?? Why Peter James and John only see Jesus and not the Voice?? Background: Mark 9: 7Jesus took Peter?

Commentaries: The apostles did not see God, as the voice belongs to Jesus who was transfigured to His Godly being.
It just confirm that Jesus is true God and true Man, yet One.

Because the voice was God the Father speaking from Heaven, and He cannot be seen.

Seraphims speaks for God but they are not visable to humans.  The purpose on the mountain of Transfiguration was to show Peter what Christ body would look like after the resurrection.   

When the Apostles  awaken from a nap, Christ the Son of God, appeared in his glorified body and was acknowledged as God's Son. Then,in a cloud covering, Christ exited and Jesus the Son of man, whom was not acknowledged as God's Son, reappeared; this seemed to some that one body transformed into the other whereas there were actually two separate bodies. This later helped Peter understand how the angels removed the stone from Jesus' tomb and extracted his earthly body so that Jesus seemingly reappeared in the Nephilim body. They were secretive about this so the world would believe Jesus rose from the dead and this notoriety would ensure his gospel be preached throughout the world.

Matthew 17:5 While he was yet speaking, look! a bright cloud overshadowed them, and look! a voice out of the cloud, saying: "This is my Son, the beloved, whom I have approved; listen to him." 6 At hearing this the disciples fell upon their faces and became very much afraid.

As they looked upon Jesus, they heard the voice of the Father, because no man can see (the fulness of) God and live. Jesus wanted them to live, and carry on His work.

The voice of God had accompanied His Glory. This is not the first time.

One cannot see a voice, by definition.