I tried to avoid the media coverage so I can't really offer a specific opinion. But while it is possible to sue members of the media for defamation, the standards for prevailing are really high. In order to overcome the protections of the First Amendment, you have to show malice to get damages, which basically requires proving that they made defamatory statements they knew to be false. That's a very loose paraphrase, and it has been years since I looked at the jurisprudence.
This may not be true here. The standard that a plaintiff show malice occurred comes out of New York Times v. Sullivan. In that case, the plaintiff alleging libel was a the Montgomery Public Safety Commissioner, a political position. In that case, the Supreme Court held that some libelous speech was protected by the First Amendment, because it was an important part of public, political discourse. Basically, journalists can libel public figures so long as they don't do it maliciously.
If I were Zimmerman's lawyer, I'd argue that in spite of how much coverage this case has received, he's not really a public figure in the sense that NYT v. Sullivan imagined. He didn't run for office, or place himself in the public arena intentionally, and therefore he shouldn't have reach that higher standard in order to show that libel occurred.
Now, that being said, truth is an absolute defense to libel claims. Even if news media creatively edited some recordings, they'd say that the recordings are what they are, and that airing them in any form can't be libelous by definition.