No audio output from S/PDIF optical connection on XPS L501X

Hi, I'm trying to connect my XPS laptop to a Logitech 5.1 system using the S/PDIF output. I get light at the far end of the cable, so I know I have the optical part of the connection working, but there's no sound. The Logitech unit tells me I'm not getting any digital data, and the internal speakers on the laptop do not turn off when I plug in the S/PDIF (as in, it acts like nothing has been plugging in). 

I've looked all over for a setting that will direct the audio to the optical output and mute the internal speakers, but can't find it. I've told the Realtek applet that I plugged an S/PDIF connection into the jack. One would think this would automatically shunt the audio to that output, but no luck. Even though it's outputting light, the laptop acts as though nothing is plugged in.

If it's relevant, the combo jack works fine when I plug in a headphone (headphones get sound; internal speakers go mute). And the Logitech system works fine with regular electrical inputs, so I don't think it's the problem.

Any advice?

Many thanks