Hello creepin,
I had the same problem (Cambridge Audio speakers with S/PDIF coax input). I did not find a solution so I bought a small optical -> Cinch converter. It's a very small device (the only problem is that it has to be powered externally). I found one on amazon for 16€ (circa 20$) and it works flawlessly.
🙂
The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.