I've just connected my TV to my DVD Home Theatre System with a Digital Optical Cable (I was using an HDMI cable before this) the DOC is so much quieter than the HDMI (as in I need to crank the volume in order to hear the TV). Am I doing something wrong? Both cables are securely plugged in...
If I am correctly guessing your setup then it is likely that the optical output has some sort of volume control or variable audio out to it. Depending on the equipment you may or may not be able to turn it off.
I guess the best way to remedy your situation is to go back to the HDMI as it carries the best available audio signal.