There is an official OST
created by Noteblock, Master Sword, and MyNewSoundtrack. And as such, these songs are all incredible.
- Once Upon a Truthful Time is initially similar to "Once Upon a Time," but the unique harpsichord notes represent a new protagonist who went looking for justice.
- Main Menu 01 has a "bouncy" feel compared to the main menu theme of Undertale.
- Enemy Approaching (Yellow) is similar to "Enemy Approaching," but the tones echo in a different theme.
- Fallen Down (Yellow) is very similar to "Fallen Down," but it demonstrates how Toriel was a kind, loving person, who cared for all of the humans that fell before Frisk.
- Ruins (Yellow) noticeably more "hollow" than its counterpart in Undertale.
- The goofy "Howdy!", unlike "Your Best Friend," reflects how Flowey is (suspiciously) helpful in this prequel.
- The slow, unique In Darkness plays as you explore a lower level of the Ruins with very little light and visibility.
- In Darkness (Genocide) is slow and mournful compared to "In Darkness."
- Main Menu 02 is a variation of the first menu theme, with references to "House Guest."
- "Justice", the "game over" theme, uses guitar notes (unlike "Determination").
- Fever Pitch!, the battle theme for the Decibat, a mini-boss, is spooky but energetic, perfect for a challenging encounter as you dodge blue and orange sound waves.
- Enemy Retreating is unique to the Genocide Route of Undertale Yellow. The motifs are slow, serious, and haunting, and the music will slow down with each dead monster until it finally fades to a low droning sound. Its main motif is also heard in some of the boss fight themes of the Genocide Route, including "Trial by Fury", "END OF THE LINE_", and "Retribution".
- The creepy Seclusion conveys the eerie atmosphere of a pitch-black cavern.
- The caverns of the Ruins begin to feel scary with the slow droning of Seclusion (Genocide).
- "Forlorn", the battle theme for Dalv, the boss of the Ruins, highlights how Dalv is not a pushover in combat, which is reflected in his powerful electric attacks.
- The calm tones of House Guest, the theme for the hangout with Dalv, reveal his kind nature.
- To Be Continued, a variation of "Once Upon A Truthful Time". The triumphant music indicates that the Underground adventures have just begun for Clover, the Soul of Justice.
- Fun-Sized is a fun theme for a secret enemy encounter: a ridiculously small Froggit known as Micro-Froggit.
- Boppin' (Bonus Track) is a humorous theme that plays during a video that demonstrates the new battle mechanics of Undertale Yellow.
- Peaceful Home is an unused track that is calm and comforting.
- Snowfall, a remix of "Snowy" (from Undertale), is more regal, but also beautiful as well.
- Drifting is an unused track for Undertale Yellow. There was supposed to be an underwater area in the game, but this idea was scrapped. The music is still relaxing to listen to, though.
- Break the Mold, a piano remix of "All-Star" (by Smash Mouth), is lonely and sad, but there are also themes of courage and willpower.
- Honeydew Snow is a cheerful and lighthearted theme for adventures in the snow.
- Enemy Approaching (Snowdin), a variation of "Enemy Approaching" that incorporates echoes, music bells, and chimes.
- "Medium" treats the player once Clover arrives in Hotland after their detour through the Dunes and Steamworks, reigniting that nostalgia through a well-made remix of "Another Medium" and letting you know that you're finally back in familiar ground, albeit for a little bit of time.
- BEST FRIENDS FOREVER and AFTERLIFE, the two themes that play during Flowey's boss fight at the end of a neutral run, turn the breakcore from Undertale's "Your Best Nightmare" up to a delicious eleven.
- "A Mother's Love", which plays in the Pacifist final boss against Ceroba, is an epic yet tragic symphonic rock theme, featuring a beautiful reprise of "Hopes and Dreams" from the original Undertale.
- "Trial By Fury", the battle theme for the Genocide fight against Ceroba, is equally tragic sounding, but this time mixed with the sheer HATRED that Ceroba feels for Clover after seeing Starlo get killed by them.
- "Remedy", which plays in the Genocide fight against Zenith Martlet, is an intense rock theme that wouldn't sound out of place in Mega Man X.
