It doesn't help make good use of the musical content, but as I have said elsewhere, avoid landfill if possible.

When we have old cassettes, or VHS videos, at church sales which have been offered for sale a few times, but failed to sell, they don't go straight to the skip.

I take them home, separate the three parts (cover(plastic), insert(paper/card), cassette (plastic/mag tape/metal)) The cover & inserts can then go in recycling, reducing the amount that goes to landfill.
I suppose I could also strip down the cassettes into recyclable/non-recyclable, but that would take too much of my voluntary time anyway.

Cheers
Nigel