A method and system accept left and right channel inputs of audio data and provide output audio data for at least three channels in a fashion that may effect a facsimile of the original soundstage on which the left and right inputs were recorded. The substrate has a first base plane, a second base plane positioned downward with respect to the first base plane and a step plane arranged in an intersecting direction between the first base plane and the second base plane. Therefore, when a first seal film is welded to the inner case, since the bag part is not bulky, the ends of the bag part are caught in a cartridge case.