I too would like to see such a solution. (permitting audio redirection from XBMC on Linux to networked device)
In my case I would like to use a Raspberry Pi at the receiver end (hooked up to a good old stereo) as it is a cheap and flexible way to receive audio through the network. With a proper digital to analog converter the sound is adequate (the RPi's on-board audio out has lousy S/N ratio)
I'm sharing my random thoughts on the subject just in case they might be helpful for others here.
Playing music on a Raspberry Pi using UPnP and DLNA :
http://blog.scphillips.com/2013/07/playi...revisited/
Also :
Volumio is a raspberry Pi Linux distribution purposed for hi-fi audio rendering on the network. Airplay is enabled out of the box. That might be an option.
As Volumio is somewhat based on Raspbian (as far as I can tell) one might be able to setup DLNA in parallel on it as well. (if it isn't already) Furthermore, Volumio has a multi-room capability, although achieved through some sort of mashup with an android app at the moment.
Another solution might be to use PulseAudio on the XBMC and also on the receiver end and configure it so the XBMC's pulseAudio sink is a network sink on the RPi's receving end. Though I do not know XBMC internal's enough to know if it might "speak PulseAudio" directly, one might be able to configure Alsa to PulseAudio to redirect accordingly.
I have no idea of the impacts on performance these solutions might incur but XBMC does have audio sync adjustments so in theory one could manually adjust audio delay to sync with video accordingly.
Anyhow, my 1.52 cents...
Cheers.