The 7-11 recording offers an exemplar of TM's voice, and also a more up-to-date image of TM than any other image the jury has been allowed to see. I'm not sure on what basis it should not be considered evidence both of the relative size of TM (certainly relevant in a self-defense claim) and his voice (relevant for identifying the screams).
Any evidence that might make it less likely that the jury would find Zimmerman guilty had to be excluded.