Elvira de Hidalgo was a popular singer in her time, but her fame as a singer has been eclipsed by her fame as Callas’ teacher in the Athens Conservatory. Just as an aside, the great Irma Kolassi was also at the Athens Conservatory and also taught Callas.
I have done something a bit unfair in this post. I have put de Hidalgo and Callas side by side. There are a couple of things to note. The sound in the de Hidalgo recordings is not great. The sound in the Callas recordings is better. De Hidalgo was a coloratura soprano. She sang principally in the resonance, especially higher in her range. Today, her sound would be considered to be too bright. Then again, listen to what we have today.
Callas was a high dramatic soprano who underwent all the Bel Canto training that de Hidalgo could throw at her. Callas’ ability to move a very big voice was due to this Bel Canto training (something that has disappeared in today’s singers). But notice that Callas does not copy her teacher; rather she makes these pieces her own, undoubtedly strongly influenced by de Hidalgo. I think that I would go even a little farther. Without de Hidalgo, there would have been no Callas.