This sketch requires the Sound library to run. To install the library:
Tools > Add Tool... > [Libraries tab] > [search for "sound"] > install "Sound" by the Processing Foundation
The sound library seems to be unreliable across systems. It will sometimes break when given an .mp3 file for playback and has a tendency to play stereo files with an immutable hard-right pan. The .mp3 problem can be circumvented with the use of a .wav file or by foregoing the SoundFile approach and using a live audio input instead (the relevant code for this is commented out in the setup() method).