Try checking teacher supply websites. Audio cassettes like that were typically made with the books for classrooms to listen to.How about this situation... Figuring out the publisher that released a certain piece of media that relate to the books you used to read.
Let me present to you this rabbit-hole that I decided to go down to because apparently I hate myself. In 4th grade my teacher had a big bookshelf in her class and three of them specifically were books regarding the unknown and supernatural.
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These three books specifically are all hard-covered and had audio cassette tapes that came in a clear cover that went over the books themselves. The weird thing about the tapes themselves is that they only told like half of the book (like 4-5 chapters if I recall) while side B of the tape had the reader of the book countdown to 1... for reasons that honestly escape me. Regardless, it's the audio cassette tapes that were my real main focus for the search.
However, and here's the kicker, NOWHERE does it show these things having the audio tape linked with them. All the sites I checked on Google (goodreads, amazon, ebay, discoverbooks, etc...) ONLY show the books by themselves and don't even mention the cassette tapes in their description. At first I thought that "Random House Inc." were the ones that made the audio tape, since the background music they used in some parts were exactly the same ones that were used in the "1988 TMNT Adventures Heroes In A Halfshell" cassette tape that came with the comic book. But then I found this...
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ONE image... on FUCKING PINTREST of all sites. Linked to a DEAD ebay link. The text on the tape is illegible, and the logo on the top left is absolutely NOT Random House Inc. which looks like this
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So I'm completely convinced I've hit a dead-end at this point.