For the youngsters reading, this is what we are talking about. Uncomfortable to use as they never sat straight, always managing to bend up on one side and down on the other. There were mini speakers in the arm rest and the sound travelled up plastic tubes. With the introduction of the SONY WALKMAN in the early 1980s, small, light and cheap headphones were being mass produced and these took over.
Someone came up with an adaptor that plugged into the socket and contained a microphone and amplifier which enabled normal headphones to be used.