I'm a bit surprised this hasn't been a more discussed/debated point. Its quite obvious the prosecution is relying on being able to show that its TM screaming as its one of the only facts (someone was screaming for help and they have a witness saying it was TM) that they mention in their affidavit for probable cause.
Both families are mostly going to cancel themselves out, though I imagine Serino will be called to testify that Tracy initially said it was not his son, so if anything, the family's statements appear to balance in Z's favor.
The FBI experts said the recording is too poor, though there are other voice experts who say they can tell who isn't screaming. The prosecution is going to fight like heck to get those experts in, but they have to overcome their own experts that say the audio isn't good enough and the voice is too stressed to try and make a determination. There of course would also be the issue of trying to compare a calm speaking voice inside a car with a screaming voice of desperation outside from far away. Also, if the screaming voice was Z's, he'd have a broken nose at this point which would also alter his voice. I'd be quite surprised if any expert testimony gets in saying it wasn't Z and I think its most likely the experts are going to say we can't tell from the 911 call.
The witness testimony is kinda mixed as many indicate the screams sound like that of a young boy. Of course there wasn't a young boy involved in the conflict, so I'm not sure how the judge/jury would see that. The defense would like to have a recent recording of TM's voice. IT doesn't seem clear if one exists. His photos would obviously indicate that he has hit and gone through puberty, but that can't show how deep his voice is, other than showing he's not boy like. W6 appears to be the best witness to identify the voice having Martin on top and yelling from what he believes to be from the guy on the bottom. Austin could help as well as he see's the man in red on the ground and he yelled for help, but Austin's mom seems to have some reservations about her sons testimony, so we don't know what he will end up testifying to.
The physical injuries seem to back Z screaming for that length of time.
The words captured on the recording also seem to help Z. They certainly sound like the desperate screams of someone in fear for their life and in pain. I guess one could argue that TM could be fearing for his life if Z had his gun drawn, but you don't hear any mention of a gun or any warning related to that in the screams. This is obviously going to be the prosecutions claim, but its pretty difficult to believe that TM and Z could have been struggling over a gun for this amount of time with Z getting hit multiple times and the gun never going off and none of the cries for help mentioning anything about a gun. Also, no witnesses ever mention seeing a gun, but obviously it is dark.
IF I had to put odds on how the court would see the screams, I'd say its right around the legal standard of beyond a reasonable doubt that Z is screaming. If you are asking what I personally believe, I'd say I'm at least 95% sure that the screams for help are Z's. IF I could hear TM's voice, it would sway my opinion.