Didier, the melodicas Lowboy mentioned are from the 1970s and are completely different from the current Hohner Student 32. The 1970s Hohners, Piano (or HM) 26, 27, and 32 models are lower in volume.
I have another suggestion, though it will work better with some models than others. If a Melodica is somewhat leaky, it will play, but at lower volume. So if you partially depress the spit valve button or lever, that will mute the sound. A few melodicas have a screw-off spit valve cover, which can be loosened. The ones I can think of, though, are expensive, the Suzuki Pro V37 and the vintage Hohner Piano-36, which is otherwise loud. But you could also just get an extra inexpensive melodica like the Student 32 and loosen screws on the reed chamber to make it leaky.