The serene sound of a distant music box... it fills you with determination.
Waterfall's SAVE Point before the second encounter with Undyne
"Memory" is the 34th track in the Undertale Soundtrack.
This music-box melody is the first instance in the game of Asriel's musical theme. The theme appears in other songs at pivotal moments of the game, such as in "Undertale" (during which the protagonist learns the past of the Dreemurr family), Finale (which provides foreshadowing to Flowey's true identity), and His Theme (when the protagonist saves Asriel).
It first plays from inside the statue of Waterfall, after the protagonist places the umbrella over the statue to shield it from the rain and complete the Piano Puzzle. It also plays later after Undyne attempts to kill the protagonist a second time in Waterfall. While the protagonist is unconscious, they experience a memory of Asriel first meeting the First Human before waking up to find that they have survived yet another fall onto a bed of Golden Flowers.
"Memory" furthermore plays while Undyne reminisces about Asgore during the visit to her house, then when Asgore offers to take care of the protagonist (assuming the protagonist has spared him after fighting him). It is next heard after the fight with Asriel, interestingly continuing very quietly even as Final Power plays and the barrier breaks.
The song plays a final time after the True Pacifist Ending Credits, on the final screen of the game where the Annoying Dog comes to the screen and sleeps. The song does not play if a Genocide Route had been completed for the first time.