It also does not indicate that no one else apart from Jesus and John heard a voice from Heaven. I would speculate that a voice from Heaven is spoken for people to hear. To be convicted.Very good...but in Jesus Baptism accounts it never says anyone heard else heard the voice....
Father, glorify thy name. A voice therefore came from heaven: I have both glorified it and will glorify it again.
The multitude therefore that stood and heard said that it thundered. Others said: An angel spoke to him.
Jesus answered and said: This voice came not because of me, but for your sakes. John 12: 28-30