Britney live in Vegas. Her vocal mic. Only her vocal mic.
If you watch the video, there's enough dancing, costumes etc. to (partially) take your mind off the vocals. But listening to just the audio is reminiscent of that infamous Linda McCartney tape.
Britney live (vocal mic only mix) - video
Britney live (vocal mic only mix) - mp3
By way of historical comparison, here is Linda singing with her hubby's band Wings:
Linda McCartney (vocal mic only mix) - Hey Jude