If it was working and nothing has been changed it's possible the DVD player itself has developed a problem (I have one that has sound but no video, so video but no sound is also possible). Consider this, particularly with an inexpensive or older player. If this is the problem it's possible you can still get sound via another connection type (e.g. analog stereo rather than optical),
On the other hand, if you have changed the connections there are several possible problems, including:
- not connected properly (check connections are firmly in place)
- cable faulty (try another cable ... unlikely since you tried this already)
- menu setting on TV is incorrect
- switch setting wrong
- faulty DVD (unlikely)
- selected an audio track the player can't handle (try another audio track).
But, you'll have to figure out which it is based on symptoms.
Hope something helps.