Okay so because Toby Fox is great at music Sans has multiple themes that all share the same root musical structure with totally different tones. Which is presumably what his MUSICAL ESSENCE will shift between based on the mood.
sans. is his base theme which is basically just representative of Sans generally being a lazy schmuck and making stupid jokes, that sort of thing. The essence of his laid back persona.
Song That Might Play When You Fight Sans is basically the uptempo fight version of his normal theme with some extra flair. It is apparently what his normal battle theme would be but he also just won't bother fighting you unless it's Megalovania levels so it doesn't actually appear in the game. SO I GUESS FOR HIM BEING ACTIVE IN A NON MURDERY WAY, which, much like in the game, is not likely to occur.
It's Raining Somewhere Else is the downtempo version of his main theme, which seems to be musically representative of his more melancholy interior life, and some of his more genuine emotions. There is also ANOTHER version of this called Sans's Secret Room which is this song but even slower and MORE melancholy that plays in the room where he has all his old science stuff and various relics of the life that, for whatever reason, he had to leave behind. INNER SKELETON ANGST.
Megalovania is the song everyone is familiar with which is the song that plays when he brutally kicks your ass for being a serial killer. It's arguable the boss theme of the player at that point more than it's even his theme song, but it would still be applicable to Sans in his No Mercy kill mode.
SANS THE SKELETON
sans. is his base theme which is basically just representative of Sans generally being a lazy schmuck and making stupid jokes, that sort of thing. The essence of his laid back persona.
Song That Might Play When You Fight Sans is basically the uptempo fight version of his normal theme with some extra flair. It is apparently what his normal battle theme would be but he also just won't bother fighting you unless it's Megalovania levels so it doesn't actually appear in the game. SO I GUESS FOR HIM BEING ACTIVE IN A NON MURDERY WAY, which, much like in the game, is not likely to occur.
It's Raining Somewhere Else is the downtempo version of his main theme, which seems to be musically representative of his more melancholy interior life, and some of his more genuine emotions. There is also ANOTHER version of this called Sans's Secret Room which is this song but even slower and MORE melancholy that plays in the room where he has all his old science stuff and various relics of the life that, for whatever reason, he had to leave behind. INNER SKELETON ANGST.
Megalovania is the song everyone is familiar with which is the song that plays when he brutally kicks your ass for being a serial killer. It's arguable the boss theme of the player at that point more than it's even his theme song, but it would still be applicable to Sans in his No Mercy kill mode.