From a ventriloquist, but I can't remember which one used it first. To open the act, the dummy would be produced from a box or suitcase on the stage. At the end of the act the dummy would say ' You can put me back in the box now' (or similar words) and the vent would open the box , lay the dummy in it and close the lid. The vent would have the dummy singing or talking and , as the lid of the box was closed, the dummy's voice would fade with the closing of the lid.Sometimes the vent would make a play of the dummy falling asleep, and would open the box slightly 'to check'.
So the image is a jokey one of someone having their say,just as the dummy and being put back in the box at the end.