Someone then linked to this youtube video:
-- Which, admittedly, is a pretty rad song.
Later on in the comments, someone noted that it "sounds like a boss battle". Well, it turns out that's because it is-- some poking around reveals that the song in the video is actually called "U.N. Owen Was Her?", and comes from a Japanese music/bullet hell game bonus boss. (See https://www.youtube.com/watch?v=Iankyw47vqY)
But wait, that's not all!
The sheet music really is just a stupid gag. Any musician worth their instrument can tell you this upon close inspection -- my brother objects, for example, to the large groups of whole notes in a row and to the illegal gliss between the bottom two staves; but he also enjoyed the explosions and the "remove cattle from stage". With the help of google image search and this post, I was able to trace it to here: