Above: a Neal Adams cover adorning one of the Power Records' LPs.
In the 1970s, if you were a TREK fan, there was nothing beyond the original 79 episodes. No TNG, no DS9, etc. So, if you wanted to quench your thirst for new adventures with the crew of the original Enterprise, you had to either go and read a book (like James Blish's 1970 Bantam paperback SPOCK MUST DIE!) or go to the toy section of the Sears store (the 1970s are pre-Toys 'R' Us) and find one of the Power Records original STAR TREK stories.
Power Records had a series of comic books with 45RPM records inserted in the back sleeve. The stories were all new, and the art, drawn by the Marvel stable (Buscema, Kane and other heavy hitters) was good. If you were a kid of a certain age, then you may remember these record/comic book combos. Power Records also had license to do these same comic book/audio adventures for all of the Marvel and DC characters. SPACE:1999 too. Later on, Power put a couple of the TREK 45rpm adventures onto a larger 331/3RPM LP, but the versions I saw had done without the accompanying comic book.
Above: a page sample from the Power Records blog.
I remember buying a couple of these and the stories were so-so, and there were off-model mistakes (Uhura was white in one of the stories, Sulu was black, for instance). The unnamed actors did a decent job, and the whole thing was done in radio drama-style, with music and sound effects. But, here too, it wasn't the music from the show or the specific sound effects. These things bothered nerdy little me. What was the point of doing TREK if you didn't care to get every little nitpicky thing right?!
So, here is the closest thing to YouTube that we had in the 1970s: a Power Records STAR TREK comic book/audio adventure story titled "A Mirror for Futility:"
Check out YouTube and that Power Records blog if you need to gorge on more of this.