I had been working in radio for 3 years when I turned 21. Did lots of radio drama shows in my College years.
I used a lot of music in my work, most often instrumental versions of popular songs. Lots of vinyl LPs that I still have. My kids about 15 years ago got me some Beatles' CDs because they discovered I owned 15 vinyl LPs of Beatles' music, NONE of it performed by the Beatles. All instrumental. Some performed on a synthesizer, some performed by Orchestras.
As other's mentioned, play lists didn't exist in 1978. We just had Casey Kasem once a week counting down the Top 40.........which ironically........I am listening to right now on Sirius XM (from November 29 1975, when I was 18).
Lots of Beatles, Monkees, Beethoven, Bach, Spike Jones would have been on my play list in 1978.