Also, tried to fix the overlapping music (I can't replicate it, so it's the kind of fix that MIGHT work, or it might also do nothing!)
I can't do the Volume settings unfortunately, because then I'd need to re-do the whole sound system to level every song to each other without using Gamemaker's built-in systems... Which would be pretty crazy, so, apologies about that!
The enemy backwards thing happens when you attack them riiight as they're turning around on the edge of a platform... I've no idea how to fix it, but thank you for reporting it!