You can hear her scream so we know that her microphone is on, she's just turned down lower than the backtrack. Nothing we don't already know because of the GMA performance of Slow Down. She's proved that she can sing the song fairly well already with her performances without the choreography.
Oh well. Bottom line is she's selling more tickets than (no shade at all.) Demi & the other HWR artists, so heavy backtracking must be fine with the general public. Britney's always done it & long touring schedules are always strenuous on the voice, especially when you don't have a super strong one.