In the book/movie Scott Pilgrim vs. The World
, a number of fictional bands show up in the story. One such band is called Sex Bob-Omb, and Beck wrote the music that they play in the film. This track is their 'theme' of sorts.
The version on the soundtrack is sung by actor Mark Webber. Beck does play electric guitar on it.
Apparently this song originally was just a short riffy garage instrumental, but the track was then chosen to be used in the film. They wanted the actor who plays the singer to do something, so according to Nigel Godrich, "Beck went away and wrote a little verse just so the character has something to sing when you see him." The song was also extended to be a little longer. (I am guessing this original version is the "fast" version that can be found on the movie's score; that's not verified though.)
One of the songs copyrighted for the movie was titled "Opener
" and I think it possible that it is this song. Just a guess though.