What version of Plex Media Player do you have installed?
Just want to make sure you have Plex Media Player (v2.46) and not Plex for Windows (v1.41.940) because PMP is only accessible through the forms now.
I believe you, but also operate on a strict “trust but verify” train of thought when it comes to troubleshooting 
As what @blim5001 stated, if you have PMP, you will need to put it into TV Mode (button at the top right) in order to see more of the audio settings. PMP is capable of direct playing those audio codecs as long as your machine supports them. I would also try an HDMI connection to your receiver if possible. I think optical should be fine for 5.1, but if you have the ability to test with other connections it could help.
