I think the audio guy was correct, and the plan was that she would sing the first song, and then lip sync the second. Everyone here seems to agree that she appears to not have her ear monitor, so that explains why she couldn't perform the first song. Then, on the second song, they probably figured she could lip sync as planned (especially since she BS'd her way through the song that she was supposed to perform), but that second song is where it became impossible to fake her way through it because she couldn't hear the song well enough to be able to lip sync it.

I'm pretty sure this was the case because it seems impossible that two different major things went wrong on the same performance.

It makes sense that either a) her ear monitor fell out, or b) they didn't play the vocal track for her to lip sync on the first song, but I can't imagine both of those catastrophic fuck-ups happening in one performance-- especially this televised performance which has to be in the top five most watched live events of the year.

But maybe she did have her in-ear monitor the whole time, and this was just a sound guy making a mistake of a lifetime... It just seems more likely that the ear piece was missing considering how little she would do vocally before starting to back off-- as though she couldn't hear herself.