ehm, a super basic one, at least in the web version (playing with widnows 10 in firefox, if it matters).
easiest one can imagine:
be on the "hub area" (you know where you to go to hunting, crafting, anything from here).
click top right save disk icon.which brings you back to main menu.congratulations, now the main menu music plays AND the music from before didnt stop playing, so you have 2 music tracks on top of each other!but this is not the end yet!
click paly button to get back into the game.click save disk again to get back to main menu!
congratulations! 3 tracks on top of eahc other.basiclaly just switch between hub and main menu via those 2 buttons to add anothe rlayer of music non top of each other , currently at layer counter 4 but I am far from my limits, must grind! :-D