Not sure if someone's mentioned it already, but Silent Death. 11 minutes of silence makes AudioSurf go really psycho, apparently.
I haven't tried this to myself, but noticed that it happened to people with John Cage's "4:33" (which features the titular amount of silence).
I figure since Audiosurf can adjust for songs ripped/recorded at quieter volumes, the intensity of the song must be determined by the maximum and minimum amplitude, and bpm's. When a song's silent or near silent, it can't determine the bpm's and just thinks its a really fast song constantly at its max volume.
Other surprisingly fast songs (due to Audiosurf guessing wrong about the bpm's, I'd imagine) are Radiohead's "Treefingers" (which is really just a bit of ambient droning) and Neko Case's "Marais la Nuit" (which is half an hour of insects chirping).