If it wasn't already obvious from its 94 metascore, Undertale is a pretty good game. Not that I was easy to persuade into buying it. Visually, it looks absolutely horrendous. But that's part of its charm, you say. Absolutely, but not to the uninitiated random person scrolling through new releases on Steam. It's good to see it's getting recognition left and right, though everyone agrees that the less you know about it beforehand the better. Go buy an indie game that looks like it belongs on the GameBoy Color without looking up what it's about? That's a pretty risky proposition if you ask me.
As of a few days ago, I've done a neutral run and a pacifist run. They were emotional rollercoasters and yeah yeah we all know what's up. And now I can't show the game to anyone or start another run because doing a true reset would break my heart after hearing the begging and knowing about the persistence tricks it pulls on you. The game is a rare example of how well meta can be done: it's funny, it's clever, and it's essential to the story.
The soundtrack is stylistically all over the place with 8-bit, 16-bit and 32-bit instrumentation mixed together, but totally amazing all the same. I don't think there's a single decidedly bad song on it, which is an impressive feat for a hundred tracks. What's fun to notice while listening to it is how certain themes are reused in creative ways throughout the entire album. And so I decided to visualize it.
In this directed graph (dot, svg) of the 101 tracks on the official soundtrack, each edge is a relation between tracks such that samples of the track with the outgoing edge are being reused or remixed in the track with the incoming edge. When determining which track should be the source and which should be the sink, the album ordering is used: the earlier the track appears on the official tracklist, the more "original" it is considered.
I created this purely by listening to the entire soundtrack and picking up on similarities, so while the majority is covered, I probably missed some connections here and there.
Edges that I deemed redundant have been removed. A good example of this is the relationship between Once Upon a Time, Start Menu and Menu (Full). The third is mostly a re-instrumentation of the second, which in turn owes its melody to the first. In my original graph, there was an additional edge between the first and third, but this (and others like it) have been removed to make things easier to read.
The most obvious recurring theme is Once Upon a Time. It's the first theme you hear when starting the game, and it's used throughout the entire game right up until the end. *Bring It In, Guys! *is a credits medley of various themes, which is why it has a lot of incoming edges.
I have no deeper analysis to go along with the graph. Just have a listen to the soundtrack and pick out the moments where the themes merge and evolve :)