Seems like some sort of DJ performance... and holograms were involved too it looks like.
Apparently, the Japanese people over at 2ch are saying that Fly! was mainly Miyahara's singing and that the multiple voices comes into play in each song with each of the women that are featured in the "chorus" section of the songs + Rachel + Nami.. so that could be why some songs sound different.
Such choral singers are:
Haruko Ohinata IMEEM link
Rin Oikawa
Fumie Masuko [no info on her, just that she's like a composer~]
Yes, the vocals have been figured out!