Maybe it's when you unlock all of the music tracks?
Like people have said previously, it's an update for unlocking all the music tracks. It's like when you do all the quests, you get the quest cape. When you unlock all the tracks, you get the music cape.
Jagex said they were working on doing that in an update like a year ago.
This is VEEERY old picture, this was found before summoning and air guitar came. Very doubtful this will ever come ingame.

