"We ourselves heard this voice that came from heaven when we were with him on the sacred mountain." NIV translation
Peter tells us that he was an eyewitness and so he knows that it is true. This is a reminder for us of how we can share the gospel of Jesus Christ and that is to simply share with people what you have seen God do in your own personal life.
"And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts." NIV translation
Peter reminds us that the Old Testament prophets spoke about things that had not yet been seen but he was an eyewitness to many of the events that they spoke about. It is a time of darkness in the world right now but Jesus is the bright morning star. In nature, the morning star follows after a night of darkness and appears just before the dawning of a new day.
"Above all, you must understand that no prophecy of Scripture came about by the prophet's own interpretation. For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit." NIV translation
Biblical prophecy does not come from man but from God speaking to the hearts of men. It is not about what the man wants but is simply delivering a message that God has for a person or persons.