Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
iTunes? (enable music sharing in iTunes menu==>Preferences==>Sharing and in the firewall System Preferences==>Sharing==>Firewall)
 
well that will allow you to choose which song from your library to play but I don't think it will play songs at the EXACT same time automatically. For instance the music server is my Sawtooth and the client is a G3 iMac across the room. When I play a song on my Sawtooth, the iMac will play the same song with no delay.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.