John 5:37 And the Father who sent me has himself testified concerning me. You have never heard his voice nor seen his form
But then we have the following incidences:
Matthew 3:7 But when he saw many of the Pharisees and Sadducees coming to where he was baptizing, he said to them: "You brood of vipers! Who warned you to flee from the coming wrath?
16 As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him. 17And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”
Matthew 17:5 a voice from the cloud said, “This is my Son, whom I love; with him I am well pleased. Listen to him!”
6 When the disciples heard this, they fell facedown to the ground, terrified.
How should we interpret John 5:37?