T'Cal, with all due respect, there is a world of difference between a book on tape (this includes each and every Trek "audio" with the sole exception of the 3 Captain Sulu audio dramas) and an audio play/drama.
The audio plays are full presentations, with full casts, sound effects, original music, etc.
The books on tape (or cd) don't even come close to matching what can be done with the audio plays.
Another difference is that the books on tape are abridgments of novels (or novelizations, whichever). The audio plays are wholly original, written specifically for that format.
To see what I mean, try listening to one of the Captain Sulu stories. One of them features the fantastic Dana Ivey in a co-starring capacity.