Routing audio input to output device

PS: If a device can route its own input signal to the output, then it's called "loopback functionality" or "direct monitoring". Because your audio interface doesn't have buttons for that, only software switches are possible.

If your audio interface has loopback, then software switches are part of the driver, and you should see them at LE audio settings.

LibreELEC supports custom alsa routing configuration via /storage/.config/asound.conf so on an OS level there is nothing to prevent you from configuring the audio hardware to route audio from the S/PDIF input to the speakers. However it cannot be done from the Kodi UI as there is simply no concept of audio input (only output) within Kodi.

The next challenge is that alsa configuration and routing is one of the Linux "dark arts" and the configuration you seek is specific to that exact hardware so there is rarely some kind of 'generic' solution or example to follow, and nobody here is an alsa expert, and Google searching for alsa configuration normally returns a million irrelevant results; it's a particularly dense and troublesome topic to search and find useful information on.

As we are unlikely to be helpful and you have nothing to lose; Give ChatGPT or GitHub Copilot examples of amixer controls and amixer contents output and some prompt/direction and ask them to suggest an asound.conf. I have low expectations of anything they create actually working, but it might give you hints for further research.