Is it possible to connect a Xbox One X to a sound card in a PC via S/PDIF?

If your sound card has an optical input (not an output), then it should work. However, you will need to check any available documentation/manuals to see if such a feature is supported by your sound card.

You can plug your headphones into your controller if you’re trying to listen to game audio. If you have the original model Xbox One controller with no 3.5mm jack, then you can get the Stereo Headset Adapter for the controller so you can plug your headphones
into it.

The Xbox does do sound processing console side (via both S/PDIF and HDMI)

Sorry, I don't understand "Only sound options on sky boxes relate to optical output so do not effect hdmi sound channel." When S/PDIF is used HDMI sound is muted (via Sky box), yes?

- I was going to suggest try S/PDIF to the Xbox and sky box. As it is a variable... it may be worth looking into.

Let me know

-Cheers

I suggest you to consult this xbox support link that comes in handy when connecting audio devices:

support.xbox.com/.../connect-sound-system

As you can see in Xbox 360 S there is the possibility to use the built-in S/PDIF (optical audio) port for the digital audio cable.

"Until then, uncompressed Dolby 5/7.1 through S/PDIF is suggested at this time through the Xbox One."

That would be quite a trick, wouldn't it? S/PDIF doesn't support UNcompressed 5.1/7.1 sound.

Hello again, the console's HDMI IN only accepts input signal, i.e. receives signals from another device.

If your console is Xbox One S or X, you have the Optical Output (S/PDIF) Out that can be connected to your Speaker if it has (S/PDIF) In.

If your console is series s or x it won't have this output.

I don't know what inputs and outputs you have in your speaker, so I don't know which connection would be best to use.