I have also experienced what I believe to be a supernatural event.
In this case, I heard several voices at once...clear as day. Two others also heard them, and it was established that nobody tricked anybody else.
Okay. In that case I would be 100% certain of one conclusion: you heard several voices at once. If your story is true, then this is the conclusion that I will accept.
However, there is no reason for me to think that this is a "supernatural" occurrence. The fact that two others also heard them would in fact make me suspect that there is a natural explanation rather than a supernatural one.