Link to Derpy_ Scientist's Official Casting Call for those who want to check it out:
https://www.youtube.com/watch?v=dHuEo_UlgPM

Applejack - Alto 2 (Lowest part)
*Bass line
Twilight Sparkle - Alto 1 (Second Lowest Part)
*Low Bahs/Percussion
Rainbow Dash - Soprano 2 B (Third Lowest Part)
*Low Ahhs/Backing Chorus
Pinkie Pie - Soprano 2 A (Third Highest Part)
*High Ahhs/Backing Chorus
Rarity - Soprano 1 B (Second Highest Part)
*Lead Vocals
And Fluttershy - Soprano 1 A (Highest Part)
*Descant/Main Backing Vocals