Connected PC to AV receiver via. optical, only stereo available

[King Mustard]

• ■MSI Z170A Gaming Pro Carbon motherboard
  ■Pioneer VSX-324-K-P AV receiver (Realtek ALC1150)

I have connected the PC to the AV receiver via. optical.

As in the receiver manual, I have selected DVD as the input then selected O1.

I am playing a 5.1 MKV but my rear speakers aren't playing any sound.

 

[americanaudiophile]

Your receiver may not support 24 bit 48khz. If it did it would not work over optical only HDMI Change it to 16 bit. Also try different sample rate. 48k is not that common.