Cassette decks are fairly complicated audio devices and they have many small moving parts. If you don't have a significant amount of experience working on electronics of this type, it's best to leave repairs to a professional, particularly if you don't have the service manual for your cassette deck. However, there are a few basic things you can do to fix common problems that don't require a degree in engineering.
Check to make sure that your Sony cassette deck is connected to the receiver or speakers and that the right input is selected on the receiver if you are experiencing problems with low volume.
Increase the volume on your receiver slowly after verifying that the cassette deck is properly plugged in.
Clean the tape heads of your cassette deck with a dry head cleaning cassette solution. These cleaners are available through most retail stores that sell home audio electronics or through numerous online retailers.
Check to make sure that the cassette is moving in the cassette deck by listening and looking inside during playback. Try another cassette after verifying that the mechanism is moving.
Try to play a different cassette in the deck if no sound is coming from the left or right channel. In some cases, the cassette can be the problem and not the cassette deck.
Check that the cable is properly connected from the cassette deck to your receiver. If so, try swapping the cable out for one that you know works.
Play back a different audio source to make sure that the problem isn't with your receiver or speakers. Try using a radio or turntable that you know is functioning properly.
Clean the tape heads with isopropyl alcohol and a clean cotton swab. Build-up of metal oxide from cassette tapes can cause one or both sides of the cassette deck to become faulty. Let the tape heads dry for 15 to 20 minutes before testing the deck after using isopropyl alcohol.
Never open a cassette deck while it is plugged in. In some cases, opening the cassette deck at all can void the manufacturer's warranty. Check your particular deck's manual for more warranty information.