Three possibilities
1. Cass Player motor is bad. Try a brand new cassette or used one you know does not squeak by playing it in another player. If tape doesn't squeak in other players it might be your player.
2. Cassette shell lubricants have worn out. (They are the side sheets of plastic on the inside of the shell. Their is one on each side of the cassette tape rolls.) carefully take apart the cassette and insert it into a new blank cassette shell.
3. Cassette tape lubricant found the back of the tape itself had worn out. (no fix possible) (although some suggest baking in oven on low for several hours) I am not suggesting this. Throw away tape and find another copy, CD , or MP3 files
MadmanJ
February 2010