According to the invention, several outward-projecting tongues are formed on the built-in housing part of a parallelepipedal built-in unit and a frame whose inner width is greater than the mounting dimension of the built-in housing, is configured on the front panel that is to be fixed on the built-in housing part. The secondary mirror also defines a second planar 46 a second concave 48 alignment surfaces, each opposite the active surface 30. The message center provides an audio message communication in a recordplayback device.