There is a way to do this, but it will be time consuming. You will need a set of tools to allow you to take apart your melodica, file the reeds, and change the reed clearance, and check your tuning.
Lower reed clearance = quieter
Higher reed clearance = louder
IOW, higher clearance gives the reed more room to vibrate up and down- raising the amplitude, literally raising the amplification of the instrument. However, if your reed clearance is too low, you may get squeaks and squawks! Conversely, if your reed clearance is too high, your instrument won’t sound until you blow hard enough to get the reed vibrating.
By changing a reed’s clearance, it will go out of tune. If you decide to, for instance, lower the reed clearance of the lower keys of your instrument, you’ll need to take that into consideration if you try to get even dynamics over the range of the instrument.
It’s also worth considering the fact that blowing harder typically means going flat, while blowing softer means going sharp (which is the opposite of instruments like the recorder).
FWIW, I lowered the reed clearance of my lower notes on my instrument. I don’t use it to play chords though! There will definitely be compromises made – I’m not sure if it is possible to get ideal reed clearance and tuning (for playing more than one note at a time anyways).
Hopefully someone with more experience can chime in!
Anyways, good luck Andy :).