AirFoil would probably be the best option.
It's essentially a plugin for iTunes It's a whole-system audio redirector that'll let you stream any audio to a whole host of destinations including Linux. It does come at rather a steep cost for a "utility" though: $25.
Failing that you might be able to run a radio-streaming app on your PC (these are ten-a-penny) and pick that up on your Ubuntu machine with a supported player. I'm fairly sure Banshee and Totem can pick up the audio.
I'm not sure these will handle more than two audio channels (or if that's even an requirement) but start your search with ShoutCast or its GPL brother from another mother: IceCast