You will need to clean the tape carriage and tape heads. Buy a small can of compressed air and a good quality tape head cleaning tape. Use the compressed air to clean out the tape carriage and the run the head cleaning tape. Use the compressed air again to clear out any dust and dirt form the head cleaning This should bring back the sound to your playback of the tapes. If not you may have other problems with the sound pick-up circuits. Check your settings and the microphone for some problem. You may have to have a service person check out the problem for you. Have them test it and give you a estimate of the cost of the repair. That way you can judge whether or not it will be worth repairing.I hope this information help you with your problem