This one's from a while ago: music mixed from the ding, chord, chimes and other sounds of old Windows operating system.
The soundtrack was made entirely in Sound Recorder. But that may be misleading, since it took more that just sound recorder to make the movie.
Structurally, the sound track is made of short clips of noises or combinations of noises. Each of these clips was made in sound recorder by distorting one of the four sounds used: ding, chord, chimes, and the Mircosoft sound.
Then sound recorder was used to mix each of these clips together into one synchronized file. Considering the amount of repetition in the song, this part was really quite easy.
Finally, the sound track was imported into Macromedia Flash, and used as the basis for the animation.