Another thing to watch out for is that many cassettes used a foam pad for the pad to hold the tape to the head. These foam pads are usually rotted and will make a mess of your tape if you try to play that tape. I recommend opening the cassettes up and look to see if it has a foam pad. If so, these can be replaced with a small piece of felt instead ( as is found in some cassettes ). Make sure the capstan is clean and that the roller is also clean. If either sticks to the tape, it will make a mess.
Of course, when opening the tape up, be careful to not unspool the tapes. When things are working well, do not store tapes that are rewound by fast forward or fast rewind. Always use the play mode to get a clean smooth wrap. Even tiny warps in the tape are much worse for digital than audio listening.
Dwight