The Dragon Knight is not sure about Elaina's claims and he is unsure whether he should follow it or just totally ignore it. But considering that Elaina has a convincing story about the song and the fact that those two important things are present there and was found, there should be perhaps a reason it was left behind there.
In the end, even though the Dragon Knight is a bit skeptical on doing so, he decided to do it and make the missing part of the music. Despite not knowing much about the music of the elves, he does know the music of dragons and like the music that was made by the elf, his music is also playable with the Harmonica.
As a result, the Dragon Knight decided to borrow the Harmonica from Elaina to create a music that can accompamy and blend together with the song Miraiya created.
