Sometimes when I go into one of my worlds, the menu music will start playing instead of the normal in-game music, and at times will play simultaneously with the in-game music. It even plays over the pause (esc) screen, and the only way to stop it is to totally close out Minecraft and re-open it. There's been a few times when it would start as normal in the start menu, but would NOT cut off upon entering a world. It doesn't happen every time, but it happens often enough to be irritating.

The only mod I'm using is OptiFine, if that counts as a mod? Otherwise, I play completely vanilla.