They CAN be rewound onto new cassette cases. I've done it once or twice when one has come unglued from it's spindle or when a mechanical part has been missing. But it's laborious...

On any older tapes I've played I've tended to hear a loud squeak.shriek continuously along with the music being played.

I know Mudcatter wilbyhillbilly has had good success converting old tapes to digital files. I dunno if that includes the situation you describe but he might have some advice to offer. See: http://www.mudcat.org/thread.cfm?threadid=88918#1680933 and http://www.mudcat.org/thread.cfm?threadid=85997#1596570.

~S~