My guess would be that when Zimmerman told the cops that he had called for help, he did not know that anyone had recorded the calls.
The only evidence that the court has as to whose voice it is calling for help on the 911 call is the sworn statement and testimony of George Zimmerman's father made under oath and subjected to cross-examination.
That will stand especially since I doubt that Sybrina will ever take the stand to defend her lying affidavit because she will be forced to answer a whole lot of other incriminating questions about her son.
And if Tracy is called, he will be asked which of his statements re his son's voice was a lie: the first one or the second one???
The Martin family have bigtime credibility problems and should face charges for lying to the police investigators.