Alfred Hertz was a conductor at the Metropolitan Opera, and can be heard in a number of Mapleson Cylinders.
Mediafire link for Mapleson cylinders
(These are zip files – left click the link, download the files, then unzip when downloaded)
As the LP notes, this is not high-fidelity sound. This is IRCC LP7006, issued in 1955, containing transfers of a number of Mapleson Cylinders, recorded live from the stage of the Metropolitan Opera House from 1901 to 1903.
Hertz can be heard on the following tracks:
5. Wagner – Tristan und Isolde – Love Duet Act 2 (two fragments)
Lillian Nordica, soprano; Georg Anthes, tenor; Ernestine Schumann-Heink, contralto. Alfred Hertz, conductor
9/2/1903
6. Wagner – Lohengrin – Mein Held, in dem muss vergeh’n (Act 2)
Johanna Gadski; soprano; Georg Anthes, tenor. Alfred Hertz, conductor
7/2/1903
7. Wagner – Die Walküre – Herrlichste Maid! (Act 3)
Lillian Nordica, soprano; Johanna Gadski, soprano; David Bispham, baritone; Marguerite Marilly, soprano; Camille Seygard, soprano; Marie van Cauteren, soprano; Mathilde Bauermeister, contralto; Carrie Bridewell, contralto; Louise Homer, contralto; Marie Maurer, contralto; Ernestine Schumann-Heink, contralto. Alfred Hertz, conductor
21/2/1903
9. Wagner – Siegfried – O Siegfried (Act 3)
Lillian Nordica, soprano; Georg Anthes, tenor. Alfred Hertz, conductor
19/1/1903