First and most importantly... you mention that your "clients" won't put up with the sync issues. This seems to imply that you want to use LMCE as a commercial offering. You need to be aware that you cannot do that without a licence for each device from Pluto. The majority of the code is released by Pluto under a GPL/LGPL however some of the critical core components are released under PPL (Pluto Public License), which for all intents and purposes is the same as GPL for amateur users, but definitely not for professional providers.... then you need a licence, otherwise you could be competing directly with Pluto with their own product!
Currently, the simplest solution to sync audio is that detailed by Andrew, using matrix switches, etc, and controlling these from LMCE. That way you are guaranteed.
You will also notice (by doing some searching here) that this has been discussed several times, including recently by me. It is not an easy nut to crack, particularly with variable rate digital media. There are possibilities that we could use some existing options within Xine, or even codec'ing the digital stream back to fixed bit rate, raw (uncompressed) format at the source, and using realtime streaming over the network to the destinations (given the client/server architecture changes I have discussed elsewhere), or even using SqueezeBoxes, as apparently they sync very well from their central server software module which can plug directly into a LMCE core.
Finally, I suggest that unless you are ready to talk specific technicalities of LMCE development that this thread may need to move back to Users. This board is reserved specifically for talking about actual development. You need to get a handle on what you are actually dealing with in LMCE both in terms of the codebase, but even more so in terms of its basic DCE device/router messaging architecture. This will shape fundamentally how you may or may not be able to approach contributing. Warning, under the hood this is a massive system, and highly complex, it bears no resemblance whatsoever to media centres like Windows, which just runs separate executables on top of the kernel/OS. LMCE is abstracted above the kernel entirely with these "virtual" devices called DCE devices each of which is responsible for controlling media playback, HA, telephony, various core services, and all communicate through a messaging bus using a specific protocol... its very distributed.