See also their History of Recordings section and explore other links of the CHARM site.
Royal College of Music (London) online exhibitions featuring historical instruments and recordings)
Personal Stereo (*navigate to p. 578* of Encyclopedia of 20th Century Technology)
For each of these, it's suggested that you follow the "see also references" or search within the same e-reference publication.
Concise Encyclopedia of American Radio
(best to click on "Read Online" at top right then search within the e-encyclopedia)
Not only music, but sound/soundscape recordings (see example: Leo T. Hurwitz, Here at the water's edge, a voyage in sound).