It's been a while since I checked back here but many belated thanks for your reply.
I actually managed to achieve something like what I was after using a clever little Windows app called DSbridge. I posted about it
here if anyone else is interested. It required using XBMC rather than Linux on Xbox though.
QUOTE(worldwhore3 @ Feb 24 2010, 01:22 PM)

from what it sounds, you have to options:
setup samba sharing on your host PC and connect your xbox to it via hardwired connection or thru a wireless router plugged into your xbox, or use a 3rd party wireless gaming adapter. doing this will allow you to connect to whatever media is hosted on the PC "on demand" if you will....
This is basically what I already had (via XBMC), but what I was after was a 'headless' Xbox housed in a cupboard and plugged into my hifi. I wanted to be able to stream audio from my Ubuntu PC without ever having to touch my Xbox. i.e. what I wanted was to be able to remote control the Xbox from the PC. In other words, I wanted to use my Xbox as a sort of cheap alternative to Apple's Airtunes.
QUOTE(worldwhore3 @ Feb 24 2010, 01:22 PM)

... or stream media from your host PC constantly, which will lead you to "tune in" to the media thru your xbox. this will allow you to only "tune in" to the media thru a sort of media streaming web server on your host PC.
as long as you have your networking information, either of these situations should be relatively painles.
This second option is exactly what I was after, but I could never work out how to do this from inside Linux on Xbox. If you have any more pointers as to how this is done, I would be very grateful. It would be great if the Xbox could "tune into" ALL audio being output by my Ubuntu PC. This would give me a lot more flexibility in terms of the apps I use to play music on my PC (at the moment I can only use Spotify and Winamp running on Wine).
BTW PulseAudio is able to do exactly this, apparently. The networked Ubuntu machines simply share their audio outputs so that a client computer automatically outputs the audio from a host computer. I've never tried it but I was hoping PulseAudio would run on X-DSL or similar.