I may be misunderstanding, I'm not seeing a JRiver audio problem here. I'm seeing a Default System Audio setting problem.
In MC, set your audio device in "Option>Audio>Audio Device" to the analogue output of the Auzentech X-Meridian. I don't have that sound card, but on just plain old Realtek motherboard audio, that device would be "Realtek High Definition Audio", or plain "Speakers". MC will then always play out through the analogue outputs, set to 4.1 via MC in your case.
Now in your PC's System Playback Devices (Windows), set your default audio device to the equivalent of the "Realtek Digital Output (Optical)", which is the S/PDIF output on the Auzentech X-Meridian. The when you play using Kodi, output to the default device, or set it specifically to output to the S/PDIF port. Chrome should be the same or similar, but as I don't use one, I don't know for sure.
Of course it could be more complex if you are trying to capture the audio from Kodi and Chrome, and send it to the JRiver WDM Driver, to be played through MC. But it doesn't sound like you are trying to do that. You are trying to direct audio out to two different ports (or sets of ports in the case of analogue), and the above should do that for you.
PS: I was going to suggest the Wiki topic on Zones as well, then realised it probably doesn't apply. You should read that Wiki article anyway, as it will give you some more insight into what MC can do.