Lets try this on for size here shall we....
John 12: 27-30
27.....This is Jesus speaking......
Now my soul be troubled; and what shall I say? Father, save me from this hour : BUT for this cause came I unto this hour.
28....Again Jesus speaking.......
Father, glorify thy name.
Now the voice from heaven is spoken of........still 28.....Then came there a voice from Heaven, saying,
"I HAVE BOTH GLORIFIED IT, AND WILL GLORIFY IT AGAIN."
29...The people therefore, that stood by, and heard it, said that it thundered: OTHERS said, An angel spake to Him.
30...Jesus answered and said,
"This voice came not because of me, but for your sakes.
Ok here are two thoughts for you.....
1...it was a different group of People who had not ever heard the voice of God in any way shape or form......both in prayer and else where.
2...People heard the voice of God but would NOT believe it was truly the voice of God.
Now lets real quick look at John 5:37-38
37.....
and the Father Himself, which hath sent Me, hath borne witness of Me. YE HAVE NEITHER HEARD HIS VOICE AT ANY TIME, NOR SEEN HIS SHAPE.
38....
and ye HAVE NOT HIS WORD abiding in you: for whom He hath sent, Him ye believe not.
Now lets look to who Jesus was talking to. He was talking to Pharisee's and Sadducees and other people and this included some higher ups too. In this time of saying you have not heard God's voice...........Jesus is referring to the fact that they pushed and believed thier jewish laws and traditions and had thier own set idea's on what was written in the scriptures and who it would be talking about.....
Here in 46-47 we see
46...
For had ye believed Mosses, ye would have believed me: for he WROTE OF ME.
47...
But if ye believe NOT his writings, how shall ye believe my words...
- Then Jesus moves on.........later down the road is when the Father spoke,"I HAVE BOTH GLORIFIED IT, AND WILL GLORIFY IT AGAIN." and the people heard the voice of God.
I hope this helps and gives you a better idea or another perspective.
Love in Christ
Jim