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