I know there are libraries like JLayer for playing MP3s in Java, but how do I implement these so the music actually plays in Minecraft? I'm having trouble getting something like this to work.
Really? But there are already like 2-3 plugins on Bukkit that can play music, and I'm pretty sure you don't need a client mod for any of them.
It's a matter of how the music works. The bukkit plugins that play music typically create a MIDI track with noteblocks or work in a fashion similar to that. Bukkit plugins themselves have almost no control over what happens on a client's machine that doesn't happen within the basic mechanics provided by Minecraft. To actually play an MP3 (or any sound file for that matter) the way that I believe you're suggesting, you would require a piece of non-standard Minecraft/bukkit code installed on the client-side.
I see, thanks. An unrelated question: How would I go about reading bukkit console output and saving it in a variable or something? If not possible with just bukkit, how would it be possible?
What output are you trying to save specifically? The server log holds everything if you're just looking for a record to browse through.
I would think so, but I have seen IO operations cache before, which would mean that there could be a noticeable delay between an event occurring and it being recorded.