There was time synths were sold with tape cassettes to give new owners an overview of what to expect, how to get set up and instructions on how to operate them. In this video, we hear the tape that came with the Yamaha DX9 FM synthesizer which kicks off with setting up the power, foot pedal and audio cables. Later instructions explain FM synthesis as well as the scientific explanation behind the result.
Another tape covering the DX7 is available here:
This may seem like a strange inclusion with a synthesizer, but in the days before youtube and the internet, this was a hugely useful way of learning features - while looking at your shiny new keyboard.