Sylvanas' Music Box, a sealed box sings dusty memories in the game, but it looked bleak and sad when Dark Queen Sylvanas sang the song. The song is to commemorate the victims who bravely against the strong monsters. The story behind the song is a sorrow of Windrunner Sylvanas: she led the Forgotten to reform the original Lordaeron to a mysterious and uncanny Undercity. Sylvanas is the High and Mighty Queen in wow game, but only this song sang her heart's grief.
Sylvanas' Music Box is a reward from completion of Shadowmourne in WLK, and it summons two Sylvanas' Lamenters to each side of the user to sing the Lament of the Highborne. Jaina's Locket, Reins of the Crimson Deathcharger, Tabard of the Lightbringer, Muradin's Favor can all get at the same time. Sylvanas' Music Box is not cheap because it can trade between players.  
Equally, Horde players can hear the Lament of the Highbourne when finished the quest: "Sylvanas' Chain."
6.1 Garrison added garrison jukebox which can listen to Lament of the Highbourne, only need you to unlock garrison jukebox, then come to Undercity royal district to find Sylvanas, pick up the box beside her and click to gain music score "Lament of the Highborne." Then you can listen to Lament of the Highbourne in your garrison.