Most full demo (no tech or sizelimit) soundtracks since the early 00s are just mp3 streams or alike, size-coded that have soft-synths or retro categories were singing is an issue due to datasize or hardware power often don't (sometimes they do as a technical demonstration).
But I did notice some 64k's and small synths-executables had singing this year, I've added small voice-samples (compressed) but that's just seconds whilst these entries had longer sequences so I'm a tad curious as well.
I wish there was more info about how they're made but I suppose part of the demo scene has always been secretive and reverse engineer it if you want to know.
The first one said made with Godot and blender so is it using premade assets? (Which I thought was not allowed)
Edit: #22 Heart of Glass says 144kB of WebGL + audio. I assume this means the audio is separate.
> I suppose part of the demo scene has always been secretive and reverse engineer it if you want to know.
Not really, there's tons of info and source code out there, but fairly scattered and disorganized. It just may not be easy to know how to search for it when coming from outside. But if you can strike a conversation with a demoscener, I never saw anyone wanting to hold back secrets, on the contrary, everyone has always been excited to talk about the magic to another enthusiast like themselves (ourselves).