Well, I have made a lot of comparisons with this issue in mind. In general, the smaller version of the same melodica will not be as loud, so that a Yamaha P-32D is not as loud as a Yamaha P-37D, and a Yamaha P-25F is not as loud as a P-32D. Usually an alto melodica will sound softer than a soprano, and the sound of the alto will be less piercing as well. Vintage Hohner “Piano” Melodicas (except for the very loud Piano 36) tend to have a more muted sound than Japanese (or other Asian) melodicas. (Among Asian melodicas, Yamahas have a more muted sound than Suzukis.)
My suggestion would be a first generation (original, metal) Hohner Piano 26. It has small keys, but smallness of keys (for whatever reason, maybe because the reeds are smaller) tends to correlate in melodicas with a more muted sound. Another advantage of the original Piano 26 is that it is easy to find one on eBay at a very reasonable price. Look for one whose photos show it to be relatively free of scratches and discoloration.
It’s probably also good to try to play softly while practicing, as a safeguard against developing a honky style.
The Clavietta would be an excellent solution, but they are hard to find, usually quite expensive, and almost always in need of repair.