1. The voice was of zimmerman
2. Zimmerman, not Trayvon, called 911.
End of story. Trayvon was trying to kill Zimmerman and any witnesses. What is the problem?
Zimmerman told the police that he was screaming for help, when he had no way of knowing his voice might have been picked up on someone else's 911 call. I think one witness, perhaps the same man who called 911, may have identified the screamer as the person on the bottom who was being beaten.
As has been pointed out before, it makes no sense for Trayvon to be calling for help when he is on top and doing the beating--the screams are not from the short interval between when Trayvon saw Zimmerman's gun and tried to grab it, and when Zimmerman used it to save his own life.
No matter what is proven in court, there will be a large number of people who will continue to believe what they believe now--the media version. Zimmerman is on trial for second-degree murder but the real charge against him is for being a racist (for which there is absolutely no evidence, but that doesn't matter...there wasn't any evidence that Alfred Dreyfus was guilty either).