From http://earlyradiohistory.us/sec018.htm As radio broadcasting began to establish itself as an ongoing public service, there were questions about the types of stations and kinds of programming they would offer. In Concerning "Canned Music Now Broadcast" from the September, 1922, Radio Dealer, George H. Fisher came to the defense of small stations like WHAW in Tampa, Florida, whose programming consisted almost entirely of phonograph records.
The networks and important radio stations had broadcasts with the well-known bands and singers of the time: Whiteman, the Ipana Troubadours, Rudy Vallee, etc. I believe the radio stations in small towns relied on recordings.
This article is relevant to the early 1920s. I don't know how much is applicable to the late 1920s.