Currently Being ModeratedFeb 20, 2013 3:35 AM (in response to timmo_0)
You can use VLC to stream audio from any source of your 10.6 imac via rtsp or http. On the 10.3 imac you may receive the stream with QT player. Setting up VLC as an stream server isn't much complicated.
Currently Being ModeratedFeb 20, 2013 5:58 AM (in response to timmo_0)
Alternatively, you could use AirPlay:
Currently Being ModeratedFeb 20, 2013 9:29 AM (in response to edex67)
Unfortunately I could get neither of these to work, probably because of my own ineptitude.
I couldn't get VLC to output from Spotify, this could be my own fault.
I also couldn't make Airplay play with OS X 10.3.9.
I did however find a messy workaround.
I am currently using Airfoil on my main Desktop to output Spotify and then am receiving it on Airfoil Speaker on my iPhone, which is in turn connected to my 10.3.9 iMacs audio input via a 3.5mm Jack.
You need to download an app called LineIn for 10.3 (http://www.rogueamoeba.com/legacy/) to play the sound from the input from the built in speakers.
To be fair it's not that messy it just requires you to have a device that can have Airfoil Speakers installed on it and have a 3.5mm Jack.