I can't play any MMO with background music on... RO, FFXI, LOTRO, FFXIV and all the ones I've dabbled with over the years, music is almost always turned off within a few days.

Though I will mention this regarding the FFXIV music... Especially during the storyline sequences (Or only during the storyline sequences, since I actually turn my music back on for the cutscenes.)... I'll hear the music and it will remind me strongly of previous FFs.