Some musical headcanons
* Papyrus’ theme ‘NYEH HEH HEH!’, was the first ‘cool battle theme’ Papyrus came up with, but he found it not cool enough to use in his actual battle, which is why he improved upon it with some percussion and extra movements and called it ‘Bonetrousle’
* Sans loves playing practical jokes - hence the ‘Song that might play when you fight Sans’ that does not play when you fight Sans. In fact, it’s never even used. Sans, you lazybones.
* As a sidenote on the above headcanon, Papyrus probably made ‘Song that might play when you fight Sans’ because he thought Sans needed a ‘cool battle theme’ of his own. (Though that doesn’t explain the bit of ‘♦☻a◘!○’ it includes...)
* Undyne’s theme has four variations (’NGAHHH!’, ‘Spear of Justice’, and then the two sped-up versions that play at the end of her date and at the end of Alphys’ date) as a musical underline to showcase how much of her character you get to know :)
* Alphys’ theme (’Alphys’) is a very halting piece that is meant to show how she stammers and how she holds herself back constantly in fear of saying something that’ll turn people away from her. When she gets passionate in her date with you, it gets really warped to show that her emotions cause her to forego all of her innate anxieties. And when Undyne shows up, it all grinds to a halt almost instantly.
* The reason why ‘Battle against a true Hero’ and ‘Power of NEO’ have the same theme is because both Undyne the Undying and Mettaton NEO fight for the same reason - to protect the ones they love. The monster community in general... and Alphys in particular. Undyne mentions Alphys having saved everyone. In fact, apart from Asgore, her father-figure and the king of the entire Underground, Alphys is the only one she mentions by name. Which, in a route where you don’t even get to see Alphys, is a pretty huge deal. And Mettaton mentioning that you probably don’t want to join his fanclub is, in my humble opinion, a reference to how he and Alphys got to know one another and bonded to be as close as possible over their Human Fanclub.
* Of all the final boss themes, ‘MEGALOVANIA’ is the shortest. But of all the final boss battles, the battle against Sans is the longest by far.
* Conversely, of all the final boss themes, ‘MEGALOVANIA’ is the only one that has nothing of the theme of the character in it. The motif of ‘sans’ doesn’t appear in ‘MEGALOVANIA’, while Flowey’s theme (’Your Best Friend’) appears in ‘Your Best Nightmare’ and while Asriel’s theme aka the Undertale theme (’Once Upon A Time’) appears in ‘Hopes and Dreams’ and ‘SAVE the World’. Probably this is meant to show that, while Flowey and Asriel both revert to who they were before they tried to kill you, for Sans there is no going back. Because you killed everyone he loved.
* and to end on a good note, ‘Bring It In, Guys!’ has parts of all the major themes, but the part that’s influenced by ‘Bonetrousle’ also hides a sneaky bit of ‘sans’ in it’s middle! You should check out the part a few seconds before Sans overtakes Papyrus on his tricycle-motor-kinda-thing (I have literally no idea what it is, if anyone does know...?)