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If you haven't already, check out page 20 of the Pioneer owner's manual on how to select that optical input to play back. You need to use the "SIGNAL SEL" button on the remote control and select "O1".
If you've done that and still no sound, make sure that you didn't leave the dust caps on the ends of the optical cable - not trying to be patronizing, but that happens. If the cable has plastic caps on the ends that fit kind of loosely, you'll want to take those off.