docs: def set_s_vol(self, vol=None, relative=False):
          '''
          Set the volume of all sounds. If @vol != None, It will be changed to 
          it (or by it, if @relative is True.)
          '''
          if vol != None:
              if relative:
                  vol += self.s_vol
              self.s_vol = min(max(vol, 0), 1)
          for sgroup in self.sounds.values():
              for snd in sgroup.values():
                  snd.set_volume()