From what we know, the Britney clip is legit. We do have a longer version but will not be posting it as of yet. less than 10 seconds ago via Tweetie for Mac
It's too distorted sounding to make anything out which sort of leads me to believe this is someone's attempt to get attention or trick us. And furthermore, the voice (what we can hear of it) doesn't sound very Britney-like.
It's only a 5 second snippet. There isn't much to make out.
....I'm still calling fake though. 
