'Parse the MusicXML <sound> element and retrieve the tempo.
If no tempo is specified, default to DEFAULT_QUARTERS_PER_MINUTE'

def _parse(self):

    self.qpm = float(self.xml_sound.get('tempo'))
    if (self.qpm == 0):
        self.qpm = constants.DEFAULT_QUARTERS_PER_MINUTE
    self.time_position = self.state.time_position
