All the cast members are actually singing with their own voices, no one was dubbed.
While filming the scene "Revenge is Gonna Be Mine," Colin O'Donoghue (Captain Hook) was performing a stunt with a chair early in the scene where he landed the wrong way and broke his foot. He continued filming though, not wanting to ruin the take, and completed the scene before admitting what had happened. Colin also proceeded to film the remainder of the season with a broken foot.
Emma's wedding dress was heavily inspired by the gown Grace Kelly wore for her marriage to Prince Rainier of Monaco.
If you listen to the tune of Emma's song; "Song Inside of Me" carefully, it is the background music that has been played continuously at certain points throughout the series. Emma really did have the song of everyone all along.
All cast members did have to "audition" in a way for their parts. But this was simply so the songs could be tailored specifically for their vocal ranges.