This is the third time recorded in the gospels that the Father spoke in an audible voice to or about Jesus (First, at Jesus' baptism in the Jordan River and second, at the transfiguration of Jesus).

This verse makes it clear that it was a voice that the Father spoke in; however, there were different reports of the same event. Some people heard a voice and thought it was an angel who spoke. Others thought it was thunder.

John 12:20-28 shows "the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned" (1 Corinthians 2:14). A carnal man with a hardened heart will always find some natural explanation for the supernatural even if he heard an audible voice from God.

A hard heart keeps us from perceiving spiritual truths and stops us from understanding. When a person doesn't understand God's Word, satan finds no resistance when he comes to steal it away. A hard heart keeps us from remembering. This isn't to say that facts or scriptures can't be recalled, it's the spiritual lessons learned that have been forgotten. 

Some people can quote scripture or remember what the sermon was about, but they can't perceive the spiritual life in it or retain what they did perceive, because of a hardened heart.

Jesus didn't need to hear this audible voice of God because he had a more sure word of prophecy than the audible voice of God from heaven (2 Peter 1:18-20). Jesus knew the "voice" of the Old Testament scriptures that spoke of the Christ being glorified and He could also hear the Father's voice in His heart as He had on so many other occasions. This audible voice didn't come to reassure Jesus, but it came to those who had ears to hear, so that they might believe.