Dampen a cotton swab with rubbing alcohol and gently wipe the tape heads--the square metal components which read the tape. Press "Play" and wipe the rotating capstan with the dampened swab. Rotate the pinch roller manually and wipe with the rubbing alcohol dampened swab. Gently wipe the stationary metal and plastic guide posts with the cotton swab. Gently dry all the parts with a clean cotton swab.
Check all the wires and input on the back of your cassette deck from any loose connections. Buy a can of compressed air and spray into the jack inputs on your deck to remove any dirt or dust that may have become lodged, thus affecting the sound quality.
Buy a demagnetizer: available from an electronic, or audio, store, or over the Internet for under $10. Plug in and switch on the demagnetizer, keeping it away from any audio equipment. Slowly move the demagnetizer toward the tape heads and gently move it back and forth over the tape heads, ensuring that it doesn't touch the tape heads. Pull the demagnetizer away from the tape heads slowly and switch off and unplug.