It's difficult to see what the jury would learn by having a bunch of Martin's friends and family insisting it was Martin, and a bunch of Zimmerman's friends and family insisting it was Zimmerman. How could they weigh such evidence?
It will likely be more complicated than just an equal stream of opposing witnesses, which will actually make it much easier to weigh, imo. As NMNM noted above, GZ has the neighbor witnesses who will likely be deemed more credible than some other witnesses because they are less interested. There may be other defense witnesses with a similar disinterest; former co-workers, etc.
Conversely, iirc, both TM's father
and his half-brother
, who should both be very familiar with TM's voice, were at best less than certain in their prior ID, with the father initially outright denying it was his son screaming. If the settlement information is admitted, it may provide a motive for changing from an uncertain ID to absolute certainty and this may outweigh the "better sound system and speakers" argument from the prosecution.
If I were on the jury, I would find Witness #6's testimony on this point, if verified by tests, very persuasive but I would dismiss the testimony of every friend and relative that says they could identify the screams as their guy. À chacun son goût.
There is little doubt in my mind that such evidence would bolster the voice ID of Witness #6 to at least some of the jurors. However, in conjunction with the other witnesses, I doubt they will dismiss his ID without that corroboration. Instead, were I a juror, Witness #6 would be added to the list of pro-GZ witnesses with no motive to tell anything but what they honestly and impartially believe. In addition, the jurors will hear the recording for themselves and likely will hear at least parts of other recordings of GZ's voice. Hopefully, they will also hear a recording of TM's suspected* much deeper voice. At least some will likely rely on their own ears in addition to the witnesses, adding their own analysis to the other disinterested witnesses.
*There is the one word recording of TM saying "hello" from the 7-11 video, and the known voices of TM's family members which sound much deeper than GZ's, suggesting his voice was likely also deep, imo.