I'm surprised nobody else has mentioned this, but have you looked at Shoutcast? It's a plugin for Winamp that lets you broadcast whatever is currently playing in your Winamp. Each of the other computers would then connect to the Shoutcast server, but as was previously mentioned, you'd almost definitely have synchronization issues, with each computer being a few seconds off from the others.
You might look into something like Windows Media Services, although that is probably more involved than you want to get. Another thing you could possibly "hijack" into working for you is Ventrilo. Ventrilo is typically pretty good about synchronization on a LAN and I think you could probably tell it to broadcase the output from you sound card instead of input from a MIC. Not too sure how the sound quality would be, but it's worth a shot.