Brian, thanks again for all your help. Spurred on and informed by your observations, I've got things working the way I want them.
My goal was to get JRiver Media Center to work with the Denon Heos multi-room wireless audio system. I've tried a number of servers, and the great control that MC gives, plus the good design of JRemote, drew me to Media Center. (The native Heos controller app is not great.)
Heos is Denon's knockoff of Sonos. I couldn't justify the expense of Sonos, but I got Heos for half price at a national retailer's sale, perhaps because there is litigation between Sonos and Denon. Also, Heos uses my pre-existing network's wifi and Ethernet to reach the whole house; each Heos speaker picks up normal wifi and has an Ethernet port. Each Heos powered speaker is Wifi receiver and a DLNA Renderer. (cf: Eustace Chisholm and the Works by James Purdy.)
I fell back to Media Center 21 because my trial period for 22 ran out, but all the controls for what I'm doing appear to be the same. We're not in the right forum now; oh well.
Here's how I got JRiver Media Center (21.0.91) on Mac OSX 10.11.6 (El Capitan) to work with Denon Heos (rev 3, 1.364.110r89781):
Tools > Options > Media Network > check Use Media Network to share this library and enable DLNA. (Go through rigamarole with Access Code.)
Tools > Options > Media Network > ... Add or configure DLNA servers ... - Generic DLNA (the default) worked for me. Audiophile 24 bit DAC did not.
Tools > Options > Media Network > ... Add or configure DLNA servers ... > Audio > Format: PCM 16 bit worked for me, also MP3 high bandwidth. Either version of PCM 24 bit didn't work (when I clicked on Play, it tried to play, then Stopped).
Tools > Options > Media Network > Advanced: DLNA Server (share your media with other DLNA devices) should be checked.
DLNA Renderer (allow other DLNA devices to control Media Center) should be unchecked. DLNA Controller (control other DLNA devices) should be checked.
The Heos powered speakers show up, after a pause of a number of seconds, under Playing Now as DLNA Renderers, grey arrows when not playing, yellow or green arrows when playing. (They also show up as music libraries, although greyed, under Playing from Main Library, even though they have no storage or library.)
You can now drag a file, album, or playlist to the speaker and it'll play. While the speaker is selected, you can use the play controls, volume control, shuffle control, etc. in the header of the MC window.
You can group speakers together by dragging one on top of another or right-click > Link Zone > Link to... and they'll play the same thing. If the speakers within a link are both audible and they're out of sync, you can select a speaker & right click > adjust link timing, although I can't figure out when (if?) that takes effect.
The Player that shows up under Playing Now plays through my Mac's regular internal speaker/headphone jack.
Now to return to our previous conversation: At that time, I was creating MC Zones, then linking a Heos speaker with an MC Zone, then linking those units together. This unnecessarily complicated attempt, which seemed logical at the time, created the problems I was asking about, which had all zones playing simultaneously through the Mac's standard output.
You mentioned sending some sounds to the internal speakers, and others to the headphone jack. I do not think these can be accessed at the same time. I think the Mac does the switching between internal speakers and headphone jack internally and mutes the speaker when headphones are plugged in.
Pardon me for being unclear. You are of course correct. The internal speakers and the headphone jack are the same thing; presumably plugging something into the headphone jack opens some contacts and turns off the internal speakers.
Mish mash of what? Do you mean web browsers, itunes, and other apps? System sounds? Or something from MC?
Was: All of the output of all of the MC zones goes to the internal speakers/headphone jack. System sounds go there too.
Is: Problem solved.
If you want to lock the system out of sound output while MC is running, you can turn on exclusive access mode.
I've tried turning on exclusive access mode for one MC zone, but then the other MC zones can't get access (to Core Audio, I guess) and give some error message and don't play. If you understand exclusive access mode better than I do, an explanation would be welcome. As in, exclusive access to what, exactly?
I have a zone for the internal sound card, which is connected to some external speakers. I call this zone "desk speakers".
OK, you set up the zone called "desk speakers", and its audio device is Core Audio, right? And then for the zone "headphones," the device is your DAC. Yes?
Maybe if you describe a scenario or two and what you'd like to happen, maybe I can guide you to a solution.
While trying to explain this, the light dawned and I did what worked, above. Thanks!
If you want to have different sounds go to different sound cards, then you'll need different sound cards.
I now agree - the way I was doing it, I made MC Zones, and they all had the Mac's Core Audio selected under Tools > Options > Audio > Audio Device, and they all played at once through the Core Audio standard output, the internal speaker/headphone jack. To avoid this I would need one device for each zone, devices such as sound cards or DACs that showed up separately under Tools > Options > Audio > Audio Device.
The way I'm doing it now, apparently the JRiver programmers can keep the different zones' digitized, packetized audio streams separate up until when they get rendered/converted to analog. My guess is that by creating zones, I was forcing this to happen on the Mac, because MC doesn't know what kind of hardware is in a zone. Doing it the way that works right, the DAC happens on the Heos speakers, because MC knows them to be DLNA renderers. I think the relevant line from Apple's "What Is Core Audio" is this: "Audio units are software plug-ins that process audio data. In OS X, a single audio unit can be used simultaneously by an unlimited number of channels and applications."
Anyway, thanks for your help, suggestions, and discussion. It made this happen much quicker and more pleasantly. I went into detail (which may or may not be correct) so that others searching the web, like I was, can have the benefit (or detriment).