THAT'S UNDYNE'S HOUSE.— Papyrus
YOU KNOW, THE ONE WITH THE SKELETON IN FRONT.
She's Playing Piano is the 82nd track in the Undertale Soundtrack. It is based on a section of the track Alphys. It plays right before the date with Undyne while Papyrus and the protagonist are outside Undyne's house.
This song seems to have been made to sound as if it was recorded live or is in fact recorded live. The track is lower-pitched in-game. It also has a slight resemblance to the song, Doctor, from the webcomic Homestuck. Even though he did not make the song itself, Toby Fox did produce the song Savior of the Waking World, which does utilize Doctor's motif.