Here are some desultory ideas:

       #) Separate the 10-20 'cheap' cassettes, the ones that are 'glued' together, and see how many have music you can't live without. Items such as "101 Banjos Play The Beatles", Myron Floren's Greatest Hits or anything by Al Martino deserve an humane burial.

       *) With the quality cassettes, the ones with screws, do a similar 'value judgement' concerning the musical content. For each one with 'trash music', take a rubber band and 'pair' that unit with a 'bad case/good music' one.

       +) Set up a proper work area with at least two incandescent bulbs and, if possible, a florescent unit located somewhat higher the others. Provision the area with proper tools such as tiny philips screwdriver, solvent-based glue for non-porous materials, a splice kit and a few 'cassette repair kits' from Radio Shack.

       =) With same-brand cassettes you can probably un-screw the case sides and transplant the reels, tape and all, into a usable case.

       ^) The cheap 'glued case' units were most likely supplied with cheap tape and are more likely to have fidelity problems. They often come a tiny block of foam plastic in lieu of the 'spring and felt' type contact pad. These bits of foam are always hard and un-usable or they have fallen-off completely. Don' try to use such cases again. Cut open with saw and pull reels and tape. If the reels can't be transplanted into a serviceable case, you will probably need to wind the tape into another case.