Lets examine her first performance, which was of Get Right, to get a flavour of things. The dance moves weren't too heavy, or strenuous, so we could logically conclude that this was choreographed with the intention of allowing for Jennifer to sing live. You can clearly hear that there are volumes changes in the vocal, where at some points are voice isn't as pronounced as others- contrast when she goes lower in her range (3.16), to when she makes her exclamations to the crowd. 4.25 also leads me to think it was live as that section of chest notes is intermixed with a shout out to the crowd that maintains the same volume and texture as the chest notes. Finally, at 4.42 Jennifer seems to suffer from a slight breathlessness that would coincide nicely with the rather strenuous movements that had just come before it.
On the other hand at 3.26 Jennifer Lopez seems to act as if the sustained note is finished, yet the sound is still being produced for a few seconds extra, and the final note of the song looks pretty hammed up (4.41). Admittedly, I was erring on the side of the vocals being fully live by the end of second song Get Loud that when she triumphantly cried out "You didn't know I could do that?" I was inclined to agree with her.
However the rest of the songs were more pronounced in there use of playback and left me in little doubt that this was an expertly crafted pre-recorded vocal that left openings for Jennifer Lopez's live voice.
To say it was a 100% live would be untrue, but similarly to say it was totally lip-synced would be unfair. It falls somewhere in the middle, probably on the side of more live vocals than less. I do think enough was sung live to show that Jennifer Lopez does have vocal chops, and though they may not be the strongest, she's certainly works the "entertainer" angle hard and well enough to compensate for any playback used.
Get Right, Lets Get Loud
Ain't It Funny (Remix), All I Have, I'm Real (Remix)
Pappi On the Floor
Jenny Form The Block (Look out for at 2.07 how the playback of "COME ON" can be heard but Jennifer actually shouts out "BRONX")
