It is understandable how a reading of Amusing Ourselves to Death, where Postman draws a sharp contrast between typographic America and contemporary electronic culture, might lead one to conclude that he was solely an advocate of books. But if you examine a bit more of his rather impressive body of work, including some 25 or so books, what you find is that he was also very much as advocate of the spoken word (and those of us that had the pleasure of knowing him knew him to be an outstanding public speaker, conversationalist, and discussion leader in the classroom).
He was a champion of rational discourse, as the highest form of discourse, involving discussions based on reading, and rooted in schooling where the curriculum was based on literacy and books (in Germany, he was at one point compared to Habermas, and indeed Postman's ideal of schooling very much parallels the Habermasian public sphere). It's the balance between orality and literacy that was achieved during the typographic era that he favored, and its loss that he lamented. That's why I've previously characterized Neil as a "defender of the word"( both written and oral), that is, defender against the rise of image culture in the 20th century, and against the supremacy of quantification that goes along with technopoly.
In America, radio before television was very much a medium for language, for discourse, for speech that was still shaped and influenced by typography. In this sense, I believe that Neil viewed radio in positive terms until television became dominant, and radio shifted in formats and styles (which he does make clear in Amusing Ourselves to Death). I also believe that much of the interest relating to radio before television's ascendancy was an interest in the nature and use of language, and this was part of a more general interest in the use of language and symbols in mass communication and mass persuasion, in newspapers, magazines, and other print media, and also in motion pictures. That's why much of the research and scholarship following the First World War and into the 60s centered on propaganda analysis (the specialty of George Gordon, who taught at NYU when Neil arrived, was a mentor to Neil's media ecology colleague and collaborator, Terry Moran, as well as my colleague when I arrived at Fordham), general semantics, rhetoric (e.g., Kenneth Burke's dramatism), and other approaches to the study of persuasion.
Radio never was a dominant medium in the way that television has been, and the internet has become. The "radio days" were part of an uneasy balance of mass media that included movies, newspapers, magazines, paperback books and, to a limited extent, record albums and live events amplified by public address systems. Much of the discussion of radio as a medium was subsumed under discussions of mass media and mass culture, and "mass man." And as noted, analysis of radio otherwise centered on the uses, misuses, and abuses of language.
Others taking part in the listserv discussion did mention McLuhan's chapter on radio in Understanding Media, and Paul Levinson's radio chapter in The Soft Edge. Also, while it's dated and out-of-print, I would also include Radio in the Television Age, co-authored by the great FM dj and host, Pete Fornatale (who now does a show for Fordham University's WFUV, and Josh Mills. Also worth mentioning is Paul Levinson's chapter on Podcasting in his recent book, New New Media, as podcasting is otherwise known as internet radio, and this constitutes a very significant development, in my view.