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Originally Posted by Callum

Many have lip synced over the years, Michael Jackson did when he performed. Performers like Whitney Houston, Jennifer Hudson and Faith Hill all lip synced when they did the National Anthem. The pregame producer actually requested them to:
http://abcnews.go.com/Entertainment/...1#.Ty_XmFyXTYg'

They lip sync because there are no sound checks and they cannot fix failed equipment because of the timeline, also because the slighest mistake would cause national upset so it's better to not sing live. Not to mention some of the perfomers who actually sang live i.e. Black Eyed Peas and The Who weren't all that great.

It's a bit different for the national anthem though, they're gonna be more concerned about getting the lyrics and everything all completely right, like the article says:

Robert Levine, executive editor of Billboard magazine, doesn't see anything wrong with that.

"If Bruce Springsteen flubs a line on 'Born to Run' only his fans will notice," he said. "If Jennifer Hudson flubs on the national anthem, people are going to get upset. People want it to be technically perfect as well as emotionally inspiring. Can you guarantee that live? Maybe. But it's good to have insurance."

I'm not sure if it's the norm for half-time show acts though, maybe it is