My granddaughter's toy is identical, and had the same rattling sound and the same free-turning crank that didn't do anything.
I watched the old Mattel youtube that showed the top being sealed the way the rim on a can is sealed. The innards are different now - they match the "what's inside" youtube rather than the Mattel one.
I cut open the bottom on 3 sides. The metal is thin so I used a box knife. As I was cutting it reminded me of opening a can of beans by the campfire, so I thought a can opener would also work but I was already done.
The loose piece inside was a metal pin about 1 centimeter long. It goes in the end of the music-box drum. Without the pin the gears on the end of the drum don't engage the worm on the crankshaft.
The jack is a sock mounted over a cardboard cone with a spring inside. This assembly comes out with a simple pull. When it goes back it fits over a circular rim under the lid. Line jack up with the lid so he's facing the way you want.
With needle-nose pliers and lots of patience I got the pin back into the drum. At first it went partway and the crank didn't work very well. I pushed it in further and the toy started working normally.
I used Gorilla tape to hold the bottom back in place. It's crude but effective, and it is the bottom.
Our granddaughter is all smiles now, and I only lost a little blood to the sharp edges, a pretty good trade. That wayward pin is only held in by pressure and good intentions, so it may work its way out again after some more drops on the floor.