The real fix is to use a RECEIVER so the audio can be decoded. Many of those optical to analog setups can't handle all audio formats.

Since you did call Sennheiser, I take it that you tested that the optical works with other gear. If you didn't that's the next step along with having someone check your box and TV settings.

Again, these non-receiver setups are hit and miss. You see folk find it not to work far too often. Then again people struggle to get a receiver working (settings and connections all the time.)