Help making Apple, JRiver and Theta work together

I currently have a Mac Mini (2015, Yosemite) running JRiver connected to a Theta Casablanca IIIHD with an HDMI cable.  Audio, for the most part is fine.  I am unable; however, to get 5.1 channel audio out of my system.  I think it's a Mac problem because I get 5.1 audio from CDs and DVDs, but whenever I look at the Mac set-up only stereo options are available (all multi-channel options are greyed out).  Can someone help me out?

Can I get 5.1 channel audio from this combination?  How?  I've tried to get help from Apple, JRiver, various forums and YouTube without success.

Is there any way to be sure I'm sending high resolution digital to the CB-III?  Most of my data are stored at 16/44.1, but I do have some higher resolution files.