How do I use the digital audio out for the C90S notebook?
The audio out jack on the Asus C90S, ProMagix x20/x25 notebook is a combo port, meaning it has dual functionality both as analog headphone out and digital SPDIF out.
To use the digital audio out on the notebook, the port requires an optical SPDIF connector, specifically a mini toslink style plug. Click here for information for cables and adapters of this type.
See also How to connect speakers to your PC.