“Gauche the Cellist” (1982) IMDb
Takahata Isao (director).
A young cellist is found lacking by his conductor, and practises exhaustively for an important performance. As he trains, various animals visit him, incidentally teaching him things.
A brief chronicle of artistic development, assisted by small animals. Based on a story by Miyazawa Kenji, who most famously wrote the basis for Night on the Galactic Railroad (1985). Took 6 years to make and does a great job of appearing simple.
References here: Ghibli movie titles.