She lip synced all the songs in the movie, so the voice you are hearing in the movie is Selena. The only part that Jennifer actually used her voice was a little sniplet during the scene of the Monterrey concert where the stage collapses and Jennifer tells the band to start the song off slow and she sings the words- "Como Laaa Florrrrr". Thats the only three words where she actually sings in the movie. I think the movie would have been horrible if JLO sang Selena's songs in the movie. She doesn't have the range or the smoothness of Selena's voice to truely represent her songs for a movie.