Art with a didactic agenda often fails by succeeding - once we learn the message, we lose interest in the messenger. Fortunately, the animal night spirits here disguise their lessons *for* Goshu as requests for favors *from* Goshu. They never explain themselves. Even for the viewer who gets the point before Goshu, the film will deserve at least a second viewing to appreciate the teacher's cunning.
Goshu begins the film as a *mediocre* professional cellist (so his playing may already sound pretty good to a child beginner!). He already shows dedication to practicing; the film carries a message more subtle than the mere value of hard work.
The spirits aim to teach the higher values of music. Indeed it takes a pretty good ear, at first, even to hear the difference they make. Sufficient ambiguity surrounds their didactic message that no one should find the film heavy handed. As subtle as its message about music itself, the movie really speaks to the art of teaching (hence didactic of the second order!).
I suspect that a Japanese viewer will immediately understand the animals as (potentially menacing) supernatural spirits (rather like denizens of dreams), and thus understand Goshu's initial violent hostility to his actually friendly visitors. If one misreads the visitors as mere (fantasy) animals, Goshu's reactions to them just seem monstrous. In other words Disney would expect you to suspend disbelief in the presence of a talking cat (not to mention a person who so quickly finds it natural to talk to a talking cat), but Goshu knows cats don't talk, though spirits, manifestations of our inner demons, very well may. A young Western child watching this film might find it useful or even comforting to have this explained in advance!