If I understand your intended configuration right, you mean to attempt to pass audio through your PC. Unfortunately, this is not something usually supported by the OS by default.
You'll likely need a sort of virtual patch cord system, software that allows you to route audio from one device to another (such as JACK Audio Connection Kit). Some possible issues: This would require that your PC is on at the times you want to pass audio through. Audio quality may suffer as the incoming sound is literally recorded on one device and played back on another and real-time, high quality digital audio recording can eat a good chunk of CPU capacity. There would be a bit of lag introduced, milliseconds worth but it can be noticeable in some situations. Some audio cards (especially the low end ones) cannot do playback and recording of different streams simultaneously (though they can often do "monitoring", allowing you to listen to a stream as it's recorded).
Another option is to use an external hardware mixer/router. These are common in the music industry but they can get pricey and I'm not able to find one that supports SPDIF.