In response to "broken music box spring":  Wind the music box and it
should eventually stop.  Do not force it when it stops.  If it does
not stop winding, then either you have a broken mainspring or the
mainspring simply slipped off the arbor or inner barrel tab; it's more
common to slip off the arbor.

If you are able to fully wind the mainspring to the point where it will
not wind any further, your issue is the mechanical movement is dirty
and needs to be serviced as the lubrication has become dry or dirty.
I hope this helps.

Kind regards,
Troy Duncan
http://www.singingbirdbox.com/