The theme song "Sakamichi wo nobotte" was specially written by Oda for this movie. He commented that he used to attend junior high and senior high at a school which was located on top of a hill and had to climb up a slope every day for 6 years. As such, when he was writing the song, the memories from that period came back to him as he reminisced about his youth. Oda also expressed admiration for the movie especially the music playing scenes which were very well done. As such, he expressed his joy to be able to deliver the passion and emotions of those who made this movie through his song.
Sources: Eiga.com / Natalie