You can check your cable box audio menu to see if the optical output can be turned back on. I have seen a similar problem with Scientific Atlanta boxes. It was fixed eventually when cable box software was updated.
I can think of two possible solutions.
1. Connect the transmitter to the TV optical audio output. Check the TV manual on how to configure it. You may have to turn it on, turn off the TV speakers, and change the audio output type.
2. If you use an HDMI audio extractor in between the cable box to TV HDMI connection it will provide an optical output.