Jesus answered and said, This voice came not because of me, but for your sakes.
Note 7 at Joh 12:30: Jesus was saying that He didn't need to hear this audible voice from God. Why not? It was because He had a more sure word of prophecy than an audible voice from God out of heaven (2Pe 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 so that those who had ears to hear it might believe.