Custom music is doable, basically you just need to find whatever music you want to replace (it'll be inside your EverQuest II/music/ folder or a subfolder thereof) and put a file of the same name inside your EverQuest II/custom_music/ folder (and potentially inside the same subfolder).
For example, I'm guessing that music box plays EverQuest II/music/freeport.mp3. To replace it, you'd put the mp3 of your choice as EverQuest II/custom_music/freeport.mp3.
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