There is only two, one of which says it was a 'boy's voice'. The other person who says its Martin is his girlfriend, who has reason to have bias. The one that was closest to the fight (the one who testified yesterday) was positive the screaming came from the guy on the bottom and he also identified the guy on the bottom as having lighter skin. Plus tons of evidence that Zimmerman was on the bottom (pics, grass stains on his clothes, etc). So who do you think is more accurate, the guy who saw them right outside his door or the woman two houses down who never saw anything and his girlfriend? Also remember that when initially listening to the 911 call, his own father said that wasn't his son.
To be honest, it's gotten to the point that I don't even think the state is trying to dispute anymore that Martin was on top and are now going on the 'he had little injuries' strategy.