Here are the parts:
HTPC: Intel NUC DN2820FYKH running Windows 8.1, all latest drivers and BIOS installed
Receiver: Onkyo HT-S3300
TV: Sony KDL-55W900a
HTPC --> HDMI --> TV --> Optical --> Receiver
I would hook up the HTPC directly to the receiver, but it's full with other devices.
Other devices hooked up to the TV like the HTPC work fine, including my Xbox 360 and DVR/Cable box.
The HTPC, however, cannot detect more than 2-channel audio. I've tried reinstalling drivers, uninstalling the audio driver and just using the default Windows driver, rebooting, etc. But every time only 2 channels are detected.
I previously had my full desktop PC hooked up in this way, and it had no problem detecting 5.1 channels. Not sure what else I can try here though. Any ideas?