I also have 'visitors' and it can be quite a shock.
Did DD give you any idea if she does have any 'friends' you can't see?

It's strange that it's only the toys playing music by themselves, i assume the jack in the box doesn't have batteries and it's one of those wind up ones, which would mean LESS chance of a malfunction.
I know DD's toys that have batteries sometimes play ALONE when the batteries are on the way out.

Hope someone has some answers for you!