So... as I posted before, a 48% match does not mean that there is a 48% chance of it being Zimmerman's voice. Let's try a different analogy here: if we compare two paintings, there will certainly be similarities in color, texture, and shape. If they are similar subjects or employ similar color schemes, then by an objective measure they may share 48% of these elements in common. Obviously, though, there is NOT a 48% chance of them having been created by the same painter. Going further with this analogy: if one were to analyze two paintings on a hunch that one is an unknown painting that should actually be attributed to a famous painter, you would certainly need more than 48% similarity in these aspects between the unidentified painting and the current sample of the known painter's work, along with testing it with other works of the painter. A 90% similarity across the board seems like it might be a reasonable standard to say that it is probably by the same person.
As far as the first part of your post, that's asinine to me and I think it would be prudent to apply Occam's Razor here. Which do you think is more likely, Zimmerman screaming for help (when it's him holding the gun) and then NOT screaming for someone to call 911 after pulling the trigger, or Trayvon screaming for help because someone's holding him at gunpoint until he is no longer physically able to scream for help?