If you save and quit from one world in single-player, then load another, there will be two music tracks playing over top of one another. One of them will continue to play even if the game is paused, and back in the main menu. Saving and quitting multiple times (like trying and dismissing different seeds for new worlds until one finds a suitable seed) exacerbates the problem. Exiting the program entirely resets the issue.
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Duplicate of MC-35856. Please use the search function next time.