Let’s stipulate for this discussion that they are from the 17 year old.
Don't know it for a fact, but in Zimerman's statement to police at the station, he said he yelled for help. And, the responding policeman said in his written statement he overheard Zimmerman (while the medics tended to him) say he yelled for help.
I’ve heard the 911 call tape maybe twice as part of news article videos. To me, and it is just my opinion, the screams are indistinct and convey mostly a sense of terrible fear. They could be “Help” uttered in a fearful voice or they could just be cries of terror. Take your choice. The one thing I did note was that the interval between the third scream and the shot was very short - seemed like just a second or two - and that would imply that they were still close enough together for Trayvon to be a danger to George when he fired his weapon.
So, as I wrote, I don’t KNOW but, as I also wrote, it really doesn’t matter whose screams they were.
It’s pretty clear from one 911 caller’s account of what they SAW that Trayvon was the aggressor in the moments immediately before he was shot. If the screams were Trayvon’s, it doesn’t matter; he was still the aggressor and still close enough to George Zimmerman that George feared for his life enough to shot when he finally gained control of the pistol. You don’t immediately switch from aggressor to victim just because you scream out of fright.
If, as you state, the screams were possibly George’s, then they simply bolster the claim that the shooting was a last, final resort after his cries for help went unanswered. But as the intervals are pretty short, could a person actually crying for help reasonably expect - especially while being pummelled - a nearly instaneous arrival of help and forego taking the first opportunity to shot? Memory, especially under stress, is a tricky thing. Did George really call for help or did he just cry out indistinctly in struggle and later think he called out for help? I don’t hear an intelligible cry for help in the 911 recording but it doesn’t matter; cry for help, gutteral cry of terror, or no cry at all, George is still the person on the ground being beaten in the moments prior to the shooting.
(In fact, Georges’s brother has stated in broadcast interviews (Piers Morgan, IIRC) that George related to him that he did call for help. He has also said that George has been very disappointed by the fact that none of his neighbors came to his assistance.)