When everything is over and Death asks, clearly expecting the answer to be "no", if she has a kiss for her granddad. Susan, borderline Emotionless Girl and Deadpan Snarker realizes that being what he is makes him more alone than any creature in the universe, actually gives him one, just before going back to her school dormitory and finally letting herself cry over the loss of her parents, who died at the beginning of the book.
In one line:
"The music had poured over (the beggars). They ignored it. Music With Rocks In was the stuff of dreams, and there were no dreams under the bridge."
Susan going back to the night her parents were killed. Death's line from the opening scene ("Yes. I could have done something.")is revealed to be directed at her, as he explains that "There has to be a law" because he can't create life, and then tells her they died instantly without pain as a desperate attempt to make her feel better.
Buddy's innocence and fierce love of music (something he had from the very beginning) makes the music's takeover that much sadder and more sinister. What he loved destroyed him... and it thought it was giving him everything he could have wanted.
On stage, when he begs the guitar to let him play his harp again, just this one time, this last time. Understanding that it'll demand everything of him in return, and pleading with it in his own accent — pleads as Imp, not Buddy — to show he's not being controlled by it, then: that sharing "Sioni Bod Da" with the world really is that important to him.
The animated adaptation makes it an even bigger tear jerker because we actually get to hear the melody, and see the reactions of a few more characters, most notably Death.
The destruction of Imp's beloved harp, the moreso in that it's such a stupid mistake that does it.
Try being a serious musician and reading that line. Every professional/semi-professional player I know has built some kind of emotional bond with their instrument.