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Ideally, the song should be the same size as the standard music.

Actually, neither the length nor the size matters much. What matters is the .ogg format. Any song you want to replace with in the game needs to be converted to this format. Then, just follow Tieru's instructions. I also recommend keeping a backup copy of the original track, in case you change your mind. Just leave the same name and add "original" or something similar to the name.

P.S. By the way, you don't necessarily need to know English; you can listen to the tracks with a player.