Playing the 1.9 version this evening (or rather, this morning... I just looked at the clock), I discovered both the music from the menu I had just left and the music in the world were playing over each other. Turning the slider to 0 turned off the in world music, but the menu music did not turn off.

Able to replicate the problem by going into a world while the menu music is playing.

I apologise if anyone else has had this exact problem in this particular version. I didn't see it when I searched.