Yeah, this takes me back! In the mid-seventies before home video got established, we used to make and trade audio cassettes of TOS episodes. I only had about a dozen or so, but I would listen to them once in a while.
Some of the episodes didn't work at all as audio dramas - "Arena" came out as mostly dramatic music and Gorn growling, while "The Doomsday Machine" wasn't nearly as effective without actually seeing The Flying Cornucopia of Death. Others made great audio experiences, like "The Conscience of the King" and "Day of the Dove". There were only a few scenes in episodes like those where I had to rely on memories of the visuals to know what was going on.