I am writing a somewhat "epic" plugin for a searchable, self-managing streaming music server for SourceMod. It includes a menu-driven interface, content searching from the "say" command line and website scripts for uploading and managing tracks. It's very large but is currently in alpha testing as I iron out bugs. I'll post more details when I feel it's more suitable for people to try it out.
To always play the same song when people join the server, just point the MOTD at a webpage with an embedded streaming mp3. Look up "html mp3 streaming" in Google for online help.
how much lag would this cause on the server though?