The
Orlando Sentinel contacted the voice identification experts. Both determined it was not George Zimmerman who called for help. Using different methods they took all the screams, put those together, and cut out everything else. You can listen to the extracted screams at the Sentinel’s site.
So an audio expert who says there's a 50/50 chance* it was Zimmerman, combined with an eyewitness who both heard
and saw Zimmerman screaming, adds up to "it wasn't Zimmerman" for you?
*48% to be exact