Hello, are couple of days ago I started using a new computer. I am not able to get 5.1 functionality from my hardware (ASUS XONAR SE using S/PDIF out. ASUS Prime Z590M - Plus motherboard with Fedora WS 34. input to a Yamaha RX-V385 with S/PDIF).
When I check Settings > Sound I select (S/PDIF) XONAR. When I test the sound only front left and front right are shown. The center, and rear speakers are not shown.
Aside from the Settings dialog not showing more than two speakers, how have you tested your audio output? This might just be a missing feature of the audio settings dialog. Disclaimer: I don’t have an audio setup capable of more than 2 channels, so I’m just guessing here.
In the recent past I have used a laptop (Fedora WS 34, HDMI to the RX-V385) to have 5.1 capability. Once I had the laptop using HDMI out, not the built in monitor and speakers (after several reboots), the 5.1 setting in sound and my TV in display would be as I had selected. I could test the 5 speakers, left (2), center, right (2).
Now I have my ASUS Prime Z590M sending HMDI to my TV and S/PDIF via the XONAR SE card to my RX-V385.