Haha I wish. No it's an ADPCM encoded wave file. The demo is like 16MB and at least 10 of that is the audio :D ADPCM is nice because it's small-ish yet very cheap to decode. The sample frequency of the file is an exact multiple of the GBA's frames per second, which meant that the entire "decode the next chunk of audio" routine could happen on the frame interrupt. This avoided audible clicks or bad synchronization, and left nearly all CPU time for the visuals. Not our idea btw, we stole the player from Janne / Matt Current.
The track was purpose-composed for this demo though, and we did have quite some fun times synchronizing the visuals to the track. We used a tool called GNU Rocket[0] to change parameters over time (eg camera position, angles, anything we'd want to animate) but it's a desktop program. So we patched Visual Boy Advance to write all parameters to some piece of uninitialized RAM on every frame, and then inside the demo we just dereferenced a hardcoded pointer to that place. No access violations in GBA land so this worked :-)
Then, when we compiled the demo for release, we simply recorded a stream of parameters, again 60x per second, as a binary blob. No compression no structure no nothing. Just on the nth frame we'd look up camerapositions[n] and that was it :)