Dudler, this may sound funny but it works. When I really got into my melodica I was playing it at all hours of the night and well I think you know how my wife felt. What I did was get a cardboard box and lined it with foam on every surface and played my melodic in the box it took about 75% of the sound and was able to close the door and not irritate anyone lol. I now have a room dedicated for practice and no longer use the box. Note if your opening on you box is to big you can put a towel over it and that helps a lot as well. It is something you need to get used to but it does work.
Monsters of Melodica