Doc is on a Mac (like me). So the Tuneblade solution, while attractive, isn't an option.
I know nothing whatsoever about macs, but my understanding was that AirPlay is what makes the whole tuneblade thing work (it's just an Airplay server for Windows). Also AirFoil (the alternative to tuneblade mentioned in that thread) appears to run on Macs.
And (forgive my ignorance) but don't Apple products have native AirPlay support? My understanding was that the Tuneblade/Airfoil solution was just a way to "export" the Airplay architecture to non-macs. I don't actually know the ins and outs, but I'm pretty sure one of the fellows in that thread (6233638) was using a variation on that solution in a primarily Mac ecosystem. He had a long post about some of it in this thread (which has lots of info about using Airplay with MC):
http://yabb.jriver.com/interact/index.php?topic=90712.msg658873#msg658873But I'm just speaking from a recollection of various forum threads on the issue; I haven't personally used a Mac since 1994
Is the long and the short of this simply that MC can't do synced playback with most DLNA devices? I'm not trying to be rude or negative in any way. Just trying to cut straight to the answer.
It depends on how tightly you define "sync," but if you mean "synced so closely that one can't tell there are two sources playing when equidistant between them" than MC can't currently do that. In my experience MC is capable of syncing two well-behaved Clients well enough for playback in separate rooms (although there will be some echo when passing between the rooms).
At the moment MC can't (by itself) reliably do echo-free synced playback to multiple output devices of any kind. Some folks have had better success than others, and folks have tried different external solutions (I've tried a few myself, like trying to use pulseaudio's RTP protocol on Linux). I've not seen any wireless solutions that worked for everyone who tried them other than airplay/airfoil/tuneblade.
In fact, just syncing two soundcards on the same PC with JRiver isn't necessarily a "sure thing" (I can often get it close, but it doesn't stay close unless I have a word-clock/ADAT connection). If you search the forums for "whole house audio" you'll find a lot of threads threshing out the sync issues.
If you want tight multi-room sync with JRiver at the moment you have two options:
1) Have a single multichannel soundcard wired to every speaker in your house, or
2) Tuneblade/Airfoil/etc.
I'm not aware of another "certain" way to do it. It's on the dev's radar for sure, and there have been some lengthy and technical discussions of possible solutions. I'm hopeful that there will eventually be a solution, but I don't know if it's on the roadmap for MC 21.
It's not a make or break issue for me as the sync (especially with other MC DLNA clients) is (as I said above) usually good enough for playback in separate rooms. There's enough drift to create an echo at transition points, but not enough that it's super obvious once you're actually in a room. It's nowhere near tight enough for playback on two systems in the same room. But this is with clients running MC (in my case raspberry pi's, and a NUC); my results with non-MC DLNA devices have been mixed (some work that well, others don't).
It's not perfect, but is good enough for parties (or at least the kind of parties I throw). YMMV, depending on the devices you're trying to sync (and your party guests)