In game Jukebox
I haven’t used it myself since I generally prefer game sounds but I understand there is a custom music option in this game and has been from the beginning. Ask in the Audio subforum for details. It lets you put in whatever music you want for various triggers, as I understand it.
if I remember right there is the ability to make the game play custom music that is not only been around since about launch but also officially supported built
yep here is link http://wiki.guildwars2.com/wiki/Customized_soundtrack