Musicracer is simply, collect some random blocks while being forced to listen to an overly loud and unpleasant cowbell noise.
Audiosurf however requires blocks to be collected in a way to form matches, these matches are the very basis of the play.
Musicracer can be described as a rhythm based game as it's play depends upon the rhythm of the music.
Audiosurf can be described as a puxxle based game, as the forming of connecting matches is a continuous puzzle to solve.