Make sure that the place it is plugged into the TV is an output and not an input for the fiber optic cable, and the place it is plugged into the Onkyo is an input and an output for the fiber optic cable. This can be connected backwards very easy. Then you want to make sure that the Onkyo is properly set on the right function to listen to the TV. Check your owners manual on how to set up the optical input and have it assigned to the function you are trying to use. Sometimes these type of receivers have an input mode button that you can set to "optical" or "analog" or "digital" or "auto". Try changing those settings after you make sure the TV optical is an "OUTPUT" and the optical on the Onkyo is a "INPUT".

After you have these thing set right you should be able to listen to it like this.

I hope this helps, if you need more help just reply back on this post, but please give more detail about the connectors and on the TV and the Onkyo, and about the digital input settings on the Onkyo. It would also help to know the model numbers of both the TV and the receiver. There is no way to check out what each of your devices are capable of doing without knowing the model numbers to look up the specs.

Thanks,

Dave