Where does 'Song That Might Play When You Fight Sans' appear in Undertale?

Song That Might Play When You Fight Sans is a red herring

The song in question is exclusive to the soundtrack. The song appears nowhere within the game itself, and is intended to throw you off from noticing the real song that plays when you fight Sans:

Megalovania, which has some special significance due to its use in other forms of media Toby Fox has been involved with.

As a side note, some mods or fangames have been created that show what a comically simple Pacifist run Sans fight would be like, some of which utilize the song as the battle music for said fight.

It doesn't.

As far as we know, in v1.1, this song is never played in game. Indeed, it doesn't even ship with the game.