Because the war in Europe made it so difficult to track down who had the rights to most operas (coupled with the studios reluctance to pay the required royalties) all the opera performed were in either public domain or were based on classical music that was in the public domain. The film makers were able to slip in a reference to the opera Faust (which is featured heavily in the original novel) by having Christine appear in the Marguerite costume as she comes off stage at the end of the film.
