Bonus Round is the track that plays when you fight Buster or Audrey as a boss.
History
v0.13: Frantic upbeat chiptune, would play as normal combat music rather than boss music. Almost entirely different from final version except for parts of the main riff.
Beta v0.4: Mostly agrees with the structure of 0.13 Bonus Round up to 0:58, but quieter and higher-pitch. Trancey mood with breakbeat from 0:58 to 1:23, then finishes with the same guitar solo as the final song. Early bits of Against All Odds play at 0:51 and 1:36. Song did not start playing exclusively for bosses until roughly Release candidate 5.
Release candidate 6: Final version. Discards techno style in favor of funk. Trumpet featured heavily.