Misc Computer Games - Undertale - Song That Might Play. Oct 18, · Song That Might Play When You Fight Sans (Undertale) easy piano letter notes for beginners, suitable to play on Piano, Keyboard, Flute, Guitar, Cello, Violin, Clarinet, Trumpet, Saxophone, Viola and any other similar instruments you need easy letters notes for. Jul 13, · Today I was was watching Undetale stuff, and one video said Gaster's Theme plays in Song That Might Play When You Fight Sans. So I checked it. It's true. So, I knew that Papyrus' theme plays inside the song too. So, I thought, Undertale takes it's hidden stuff seriously. I mean MORSE CODE, HIDDEN FILES, HACKS. And then a random song appears in the soundtrack.