Probably check to see what format the computer is sending sound over in. Sonos can read stereo, or Dolby Digital, but not Dolby Digital Plus or DTS.

You may need to check that the optical output on your sound card is actually turned on, too. Several of the ones I’ve interacted with require some messing around in order to make it actually an active output. You know it’s working when there’s a red light coming out the end, then you can start messing with the codecs it is using. 

looks like this is the card that I have:

C:\Program Files (x86)\Realtek\NICDRV_8169\win10

Great. Check the manual. This is a device I’ve never used, 

I believe it is turned off because I do not see the red light.

Here are the settings, though for encodings

Looks good to me. You may want to check the manual, or a support board for Realtek to figure out why the port is not actually active. I know much more about the Sonos ecosystem than what feeds it.

This is the motherboard I have: https://www.laptopsdirect.co.uk/asus-tuf-gaming-x570-plus-atx-motherboard-socket-am4-usb-3.2-gen-3-tuf-gaming-x570-plus/version.asp?refsource=ldadwords&mkwid=sXkys9Vx7_dc&pcrid=444011661573&product=TUF%20GAMING%20X570-PLUS&pgrid=104498047833&ptaid=pla-786108720176&channel=googlesearch

Asus Tuf Gaming X570 plus

The sound card must be integrated

found the name of the audio card in the motherboard manual:

Realtek S1200A 8-channel High Definition Audio CODEC

And the port that I have stuck the Sonos in is indeed an optical S/PDIF out port.

As the audio chip is on the motherboard, I would check the bios settings on PC startup .. check that the S/PDIF port is enabled.

Looking briefly at the manual, perhaps check "Advanced/Onboard Device Configuration" to see if that has any audio options that need to be enabled.

so! Turns out I have to fire up the Sonos app now it works. The only issue is that is requires 2.4 GHz wifi, which is OK. I like to have it on 5 GHz though