Millie's singing was dubbed by Australian-Mauritian opera singer Stacey Alleaume, who has won several prestigious awards in Australia and New Zealand. Max's singing was dubbed by Nathan Lay, also from Australia, who has also won numerous awards and contests. The only live recorded voice in the film was Andrew J. Carter's as Fergus McKenzie, as his role required him to 'crack' which could not be overdubbed.
Danielle Macdonald (Millie) told interviewers that Joanne Lumley, who played the opera coach, never needed a second take.
Despite not doing the actual singing in the film, Danielle Macdonald and Hugh Skinner needed much training for their roles. They had to learn the songs and take voice lessons, so that it could be dubbed easily. Macdonald spoke of how terrifying it was for her to simply sing badly and not worry about how she sounded.
Joanna Lumley is married to classical conductor Stephen Barlow, who is credited as Opera Consultant on this film.
Danielle Macdonald (Millie) previously played an aspiring musician in Patty Cake$, in which she played an amateur rapper.