The music on this piece is from 3 movements of Mussorgsky’s
"Pictures at an Exhibition". Played on a light-beam emitting pipe organ with drums, rock bass, many other instruments, and some pyrotechnics thrown in at the end!
The cathedral structure and major instrument groups are faded on one at a time to reveal the construction of the set, concluding with some unusual camera angles not used in the actual animation.
For the audio track a distant organ plays the music from Mussorgsky's "Promenade".