I think a plugin where you could drag music (mp3) or other sound codecs into a folder and it could play them in your server. Also It would be cool to have a setting to only play in certain WG regions, and announce what is playying if your in radius of sound. EDIT: Also players should be able to toggle music off or on with a command /musictoggle
Not possible without a client mod, alternatively you can open a web server for people to tune in. Once they are in the region, the music will play on the browser.
There are plugins like this for Spoutplugin + spout client but you either to use version 1.3.2 or wait until spout team finish there 1.4 version (Fingers crossed this is a matter of a days/weeks) Jukebukkit Ambientspout and umm err there's one more out there who's name escapes me. keep in mind that people could still connect to a spout plugin enabled server without spout. they will just not hear the music without the spout client. (so you're not making anyone miss out if they dont want to use a different client) EDIT by Moderator: merged posts, please use the edit button instead of double posting.