Foreground/Background Operation
 
  Operation   Action
  MF          Sets music (PLAY statement) and SOUND to run in the
              foreground. That is, each subsequent note or sound will
              not start until the previous note or sound has finished.
              This is the default setting.
 
  MB          Music (PLAY statement) and SOUND are set to run in the
              background. That is, each note or sound is placed in a
              buffer, allowing the BASIC program to continue executing
              while the note or sound plays in the background. The
              maximum number of notes that can be played in the
              background at one time is 32.