Sans.

"sans." is the 15th track in the Undertale Soundtrack. It plays during most conversations with Sans, inside Grillby's, in Napstablook's snail farm, and throughout the entire game.

When sped up, it sounds remarkably similar to "Song That Might Play When You Fight Sans

About 35 seconds into It's Raining Somewhere Else (Sped up by 33% from the beginning) and then play sans. Then it sounds very similar.

Trivia

 * The track appears to predate Undertale's development, having shown up as background music for a Homestuck-related project as early as October 2012, as " ", explaining its filename in Undertale.
 * This track was used as background music for Undertale's Kickstarter video, along with some others.

Sans. Sans. (Música) Sans. ‎Sans. Sans.