Combined with a few more pesky details:
1. Witness reports have TM on top, beating Zimmerman.
2. Zimmerman has head wounds and broken nose, TM's only injury (per the mortician) is the gunshot wound which abruptly ended the beating he was administering to Zimmerman.
There's no logical reason the guy on top is the one screaming for help. TM's own father originally said it wasn't him, and then changed his story later on. On the other hand, the "experts" who claim differently are getting paid to contradict the obvious.
I'm not overly concerned with what his father says, to be honest. I lost a son myself three years ago, and I would give the parents a lot of leeway considering the emotional strain they are under. I don't grant the same leeway to the people trying to exploit their grief.
As for the voice recognition part, I'm inclined to think the father's first reaction is the more credible, for the simple reason that I believe it was given in a less politically charged context.
Having said all that, I also think that the parents' statements, in general, could not possibly be less relevant, since neither was a witness to anything, and I don't think there is any point in debating them one way or the other. They are speculating, just like the rest of us.