Once Upon a Time
The title theme, and the seed most of the game's other melodies grow out of. Understated on first listen, load-bearing by the time you've heard where its phrases resurface.
Soundtrack
Toby Fox wrote, arranged, and largely produced the entire score himself, and it carries more of the storytelling than most games ask their music to. Leitmotifs travel: a phrase you hear as a cheerful aside in Snowdin can return, slowed and re-harmonised, as the saddest moment in the game. A short, incomplete list of the tracks players keep coming back to.
Theme
Snowdin
Snowy
Snowdin's town theme: a plain, cosy chiptune loop that matches the area's low-stakes puzzle-and-banter pacing better than anything more dramatic would.
Battle
Bonetrousle
Papyrus's boss theme, built on a bassline he apparently composed himself in-universe. Upbeat, a little chaotic, entirely in character.
Waterfall
Quiet Water
The ambient theme threaded through Waterfall's echo-flower corridors. Sparse and reverberant on purpose — it leaves room for the area's dialogue to land.
Battle
Spider Dance
Muffet's theme, waltzing between playful and menacing in a way that mirrors the fight itself. One of the score's clearest examples of music doing a boss's characterisation for it.
Battle
Megalovania
Sans's theme, and by a wide margin the track that escaped the game entirely into general internet culture. Fast, relentless, and built to match a fight that plays by different rules than everything before it.
Finale
Hopes and Dreams / SAVE the World
The Pacifist route's closing battle theme, stacking nearly every major leitmotif from the rest of the game on top of each other. It only works as well as it does because you've heard every one of those pieces separately first.