Set the volume of all sounds. If @vol != None, It will be changed to 
        it (or by it, if @relative is True.)

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()