Can you view a shared iTunes 11 library from iTunes 7, or use iMac as an Airplay target?
Install Airfoil Speakers on the G3.  They have a legacy version that runs on 10.4.

http://www.rogueamoeba.com/airfoil/speakers.php


  Run it on any Mac and it will immediately be able to receive audio.
  Airfoil Speakers for Mac receives audio from Airfoil for Mac and
  Airfoil for Windows, as well as iTunes, iOS devices, and other
  third-party software. See it in action.