Undertale

Hidden Melodies

  Undertale has one of the most extensive soundtracks of any video game I have played. Toby Fox composed the entire soundtrack himself, and often reused melodies that appear throughout the game to induce nostalgia in the player (reusing melodies also made it a lot easier for him!). The melodies are used intentionally and cleverly, and are very meaningful to the situation in the game. Some of them are really obvious. For instance, the three melodies used in the game's theme song, "Undertale", appear in about half the soundtrack, including:

"Undertale""Once Upon a Time""Home""Hopes and Dreams""SAVE the World""Reunited""Hotel""Can You Really Call this a Hotel? I Didn't Receive a Mint on my Pillow or Anything""His Theme""Memory""Menu""Start Menu""The Choice""Fallen Down (Reprise)""Bring it in, Guys!""Last Goodbye""Good Night"

Some other ones are obvious, such as boss remixes of characters' general themes. "Bonetrousle" is a remix of Papyrus's main theme, "Nyeh Heh Heh!" "Spear of Justice" is a remix of Undyne's introductory music, "NGAHHH!!" "Power of NEO" is obviously a version of "Battle Against a True Hero" (to make the player think it's going to be another epic boss battle, but it's not). All the tracks on the soundtrack are slowed down in Genocide.

"Bring It In, Guys!" is the credits song, and so includes versions of almost all the main themes in the game. ("Enemy Approaching", "Nyeh Heh Heh!" "sans.", "Snowy", "Dating Start!", "Spear of Justice"/ "Ruins", "Death by Glamour", "Another Medium"/ "CORE", "Bergentrückung", "Once Upon a Time", etc.)

Some repeated melodies are a little less obvious...

"Dummy!" and "Spider Dance" are remixes of "Ghost Fight". "Pathetic House", Napstablook's house theme, is part of the same melody slowed down. Napstablook remixes music and shares it online; maybe his songs are the new melodies that he remixed himself to make them his own.

Listen to the beginning of CORE. Now listen to the beginning of "Death By Glamour". "Death by Glamour" follows CORE's melody for approximately the first half of the song. It also includes "It's Showtime!" (which is obvious), and, more subtly, the middle part of "Metal Crusher".

"Your Best Friend" is Flowey's theme, and appears symbolically in many different places (surfacing unexpectedly as Flowey does). "Your Best Nightmare", the Photoshop Flowey music, obviously includes it. "Finale" (which plays near the end of the Photoshop Flowey battle) is a remix of it. "Your Best Friend" also appears in "Hopes and Dreams" and "SAVE the World" (because Asriel is Flowey). In the Genocide Run, "But Nobody Came" is a dramatically slowed down version of it.

"It's Raining Somewhere Else" is a version of "sans." Sans' battle music was originally going to be a remix of it, but "MEGOLOVANIA" was used instead. His original battle remix is still included on the soundtrack under the title "Song That Might Play When You Fight Sans".
"Song That Might Play..." also includes part of Papyrus's theme, and in the background, Gaster's theme.

"Dogsong" is a version of (a part of) "Enemy Approaching". "Tem Shop" is a version of "Dogsong". "Anticipation" is the background motif for "Enemy Approaching".

"CORE" is a variation of "Another Medium".

"Waterfall" is a slowed down version of "Ruins". "Quiet Water" is a version of (part of) "Waterfall" (and therefore "Ruins"). The chiming background motif in "Waterfall" is also the background motif of "Another Medium" and "CORE".
"Don't Give Up", "Respite", "An Ending", and "But the Earth Refused to Die" are also slow versions of "Ruins" and/or "Waterfall.
The "Ruins" melody is part of "Spear of Justice"/"NGAHHH!", and appears in "Battle Against a True Hero".

"Run!" is a sped up version of the triumphant opening riff in "Spear of Justice".

"Dating Start!" and "Dating Fight!" are versions of "Shop". "Confession" uses the same melody, which also appears in "Hopes and Dreams" and "Reunited". This melody is a variation of "Snowy" and "Snowdin Town".

"Final Power" is a slowed down, reversed version of (part of) "Hopes and Dreams". This part is a remix of the last melody in "Once Upon a Time".

"Live Report" and "Death Report" are versions of "It's Showtime!"
"For the Fans" is a version of "Oh! One True Love" and "Oh! Dungeon".

"ASGORE" includes "Bergentrückung", "Determination", and Toriel's boss theme, "Heartache".

The song that plays when fighting Snowdrake's mother (the Amalgamate) is a disturbed, out of tune version of "Snowy".

"His Theme" appears in "Finale" as the chiming bells in the background.

"Burn in Despair!" is the song that plays when you fight Asriel as the God of Hyperdeath (after "Hopes and Dreams", and before "SAVE the World"). It is a remix of "You Idiot", which plays right before the Photoshop Flowey fight. "Your Best Nightmare" is also a remix of it.

"The Choice" is a dramatically slowed-down version of (part of) "Undertale". (It's slowed down by 666%. Make of that what you will.)

"Barrier" sped up is the breathing sound in "Amalgam" (it originally comes from EarthBound).

The sleepers in Snowed Inn snore the song "Determination". It's pretty creepy.

Even creepier is the song that plays at the end of the Soulless Pacifist Run, where "Memory" would normally play (it also plays during the "fight" with Monster Kid before the Undyne the Undying boss battle). It's a dramatically slowed down version of "Anticipation."

"She's Playing Piano", which plays right before Undyne's date, is part of Alphys' theme, hinting at Undyne's feelings for Alphys.

And my favorite: The True Lab theme ("Here We Are") is a darker version of Alphys' theme.

The name "MEGALOVANIA" is a combination of 'megalomania' and 'Transylvania'. The song was originally composed for EarthBound's Halloween Hack (which Toby Fox created). It was also used in Homestuck. "MEGALOVANIA" was one of six songs originally composed for other projects, including "Fallen Down" and "Another Medium".

(For the record, I don't know if this one is intentional or not, but the first three notes of the "Once Upon a Time" leitmotif and the first four notes of "MEGALOVANIA" are the same)

Of course, we all knew Toby Fox was brilliant. The number of things he thought of for this game is insane.