You're looking for SlimServer, which fortunately is an Open Source application.

Basically it will be a streaming music server on your LAN that will look through the folders of your choice to add to its music library. It then allows you to password protect the streams, and log in to the server to change the song, etc. Your pc-server could have some music player running that connects to the stream-server application to play that, so when you change the song via the interface from any pc, your pc (and any others) playing the music from that stream reflects this change.

I'm not 100% sure if you can have it let you download the files, but you could always have Apache on a directory with all of your music for that part of it. Though I'm not sure if you can run them together...you'd have to change the default ports for one of them I think...


Cheers.