No, they give detailed instructions to the editors where they want the cuts to be made. My nephew is a film and video editor who worked for CNN for a while. He recently quit his job at a local TV station. He got a better job with a film maker, but he also said he couldn't stand the news business anymore. He said that they're all on a mission to change the world.
He told me there is absolutely no way that the Zimmerman 911 tape was edited by mistake, and claims that it was are bald-faced lies.
Reporters in the field sometimes do edit their own audio (not video) for a report. I work in media and have helped reporters edit audio.