I just stumbled across an app called Airfoil that does sync audio to Airplay devices (Airport express, AppleTV etc) and PC to PC from JRiver MC18.
I'd been searching for a solution to this for several months and nothing came up in the forum search so I thought I'd post my experience and setup so others can benefit.
I tried linking zones and it was problematic and never stayed in Sync for me so I needed another solution
1st my setup
Lenovo M72E Tiny HTPC with external esata RAID using HDMI to receiver in main lounge
This is the main Library and Server
I have 2 zones setup
1 for standard local HDMI output and the other zone called Multizone using the Airfoil audio driver
2 PCs in other rooms with MC18 and Airfoil speaker installed
iPad and iPhone with JRiver for remote control and independant zone playback
You need to install Airfoil server on the main library server, in my case my HTPC and then create a new zone called multizone in MC18.
Set the audio output in MC18 for the multizone to the Airfoil audio driver called "Speakers (System Audio Driver)"
Set the source to MC18 in Airfoil
Under Preferences in the Airfoil Server select your local preferred output device, im my case the HDMI output. This connects the chain from the Airfoil Driver in MC18 to the output from Airfoil to your usual audio output device.
Run the Airfoil Speaker App on your remote PC
Start a song on your main server to the multizone zone and click on the button in the Airfoil App on your server to activate the PC zone
If that works then your pretty much set and can just enable other Airplay enabled devices in the Airfoil App and it will sync your audio to the main server, your PCs and Airplay devices.
You can set Airfoil to "Instant On" so when ever you start Airfoil server it will automatically start MC18.
I also have the ipad and iphone with JRemote and connect to the main server to control overall volume, playlists, playback etc from anywhere in the house.
I also set MC18 on the other PCs to the MultiZone zone from the library server under playing now so I can have theatreview of whats playing through the house on multiple screens or control the playback through the whole house from any PC.
And I also have an Apple TV setup in another room and it works flawlessly across all devices in Sync. There is sometimes a couple small hicups when it first starts playback for the first 2-3secs until its buffered but once its going it works fine, including crossfading and skipping songs with crossfade!
It sounds complicated but its actually really simple. If you understand zoning in MC18 you should get this going with no problems.
I had to set the Audio device to "Speakers (System Audio Driver)" in MC18 for the Multizone zone to get it to work otherwise it would crash Airfoil when I started playback.
If your only interested in 2channel audio then theres no need to create another zone, just set your default zone audio to the Airfoil audio driver called Speakers (System Audio Driver).
When I have some more time I'll post some pics of the various settings for the technically challenged as I know instructions can be hard to interpret on forums without pics!
PS.. there are also Airfoil Speaker and Airfoil Remote Apps for iPhone/iPad to change independant zone volumes and have sync playback from MC18 on your iphone or ipad!
Airfoil also works with Macs... and Airfoil remote for IOS iphone/ipad currently only works with macs. The Airfoil Speaker for iphone/ipad needs a good strong signal too.