I'm not sure if it is the case for this jam but generally fixing a bug is allowed in the rating period. So if you had some music ready but it is not working I don't think anybody would blame you for adding it.
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Nono I just never got to it, lost too much time with the other parts of the game and with life stuff. I haven't even started producing music/sounds. It is a real pity though, I was thinking of adding sounds made by myself like "Crash" or "crack" with comic style explosion boxes when the box hit the ground. About the first part, it is also possible in this jam to fix bugs if you keep the original build available and explain the fix (Jerbob confirmed it in the discord server).