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.
Hohner Piano 26
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.