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

def set_m_vol(self, vol=None, relative=False):
        '''
        Set the music volume. If @vol != None, It will be changed to it (or by 
        it, if @relative is True.)
        '''
        if vol != None:
            if relative:
                vol += self.m_vol
            self.m_vol = min(max(vol, 0), 1)
        pygame.mixer.music.set_volume(self.m_vol)