Even if that is what Z said, he never said it to T. If we can barely make out what Z was muttering, then how on earth could T have hear it if he was not even close by.
There is a difference, especially if this comes before a jury, between someone calling another person a random bad word - like a-h, and someone of another group calling you a racial epithet. They are likely to understand hitting someone who calls you a dirty racial epithet.
Mind you, not pummeling the person to death, just a punch or slug.
But as racially inflamed as this already is, this tape does NOT help Zimmerman. It may show he was heading toward trayvon with racial malice.