 According to Terry Gilliam (qv)s commentary on the Criterion DVD,  in the scene where Raoul and Gonzo raise havoc at the  Debbie Reynolds (I) (qv) concert, the voice heard in that scene  that is supposed to be Reynolds singing actually IS Reynolds.  Gilliam is friends with Carrie Fisher (qv), Reynolds daughter,  who spoke to her mother about recording a couple lines for the  movie, and Reynolds agreed.