A video that has appeared on the Ebaum's World website claims to have isolated the sound of Britney Spear's microphone from all the other voices and instruments that appear on her 'Live From Las Vegas' DVD, which was released in 2002. As you probably won't be surprised to find, it shows that dancing energetically makes you out of breathe. Still, even allowing for that, the singing is pretty poor.
Watch the video here