Here’s a quote from a Bernard Hermann biography (Hermann scored Citizen Kane and composed the aria sung by Susan Alexander):
“If Susan couldn’t sing at all, then we know she wouldn’t have found herself in this position. But she had something of a little voice. So I wrote this piece in a very high tessitura, so that a girl with a modest little voice would be completely hopeless in it. … We got a very charming singer [Jean Forward] to dub Susan’s voice, explaining to her the purpose of the effect. Notice–the reason Susan is struggling so hard is not that she cannot sing, but rather that the demands of the part are purposely greater than she can ever meet.”
Other online sources identify Jean Forward as a lyric soprano with the San Francisco Opera.
Susan is forever identified with Marion Davies because Kane is identified with Hearst (and they also shared some superficial characteristics, such as a penchant for jigsaw puzzles), but there are much more direct historical parallels to her opera “career.” Welles himself claimed that the character was based Ganna Walska, wife of tycoon Harold Fowler McCormick, who set her up in a lead role at the Chicago Opera despite a voice that only he could love. Also see Gladys Wallis, whose husband built the Chicago Civic Opera House for her.