Dear Center for Social Media,
I'm a radio reporter, and I was interviewing some protesters at a rally where the police had given the occupiers an ultimatum. Behind my interviewee, you can hear some of the protesters are chanting, “All we are saying is give peace a chance.” Actually, it's more like they were singing that line from the John Lennon song. This is pretty much the best interview I've got. Do I have to use another interview instead? Yoko Ono is known to be really litigious.
You're in luck, because other journalists have helped you out, by creating a Set of Principles in Fair Use for Journalism . The very first situation is "incidental and fortuitous capture of copyright material in journalism.” That's what happened to you, as you describe the situation. So your problem is fair use-eligible.
Now check each of the limitations:
Did you already agree with anyone not to film any part of the Lennon song at the event? (Probably not--that kind of thing happens sometimes when reporters cover a concert or go on a movie set, though.)
Did you ask the protesters to sing that song?
Did you specifically capture the Lennon song performance in order to give listeners the pleasure of a musical experience?
Should you attribute it?
You can make your own decision, knowing that fair use does apply to incidental material and that these are the questions your peers think you should ask.