Mezza voce/sotto voce

Considering the definition that we most agree with (mezza voce = "in a half voice", sotto voce = "under the voice"), I'd say that when we play a Chopin tune in a cantabile way, the voice is much more prominent than the rest of the piano writing. Mezza voce goes against this "proeminence": the tune is not that much prominent. Sotto voce is even less prominent than Mezza voce, or in a "mysterious" way, according to what Stu has said.

What are we going to do in order to achieve these effects depend on the musical context, the texture of the writing, the register we are playing and it even depends on the composer style. I know Chopin uses mezza voce a lot, and it was mistakenly written as mezzo forte in some editions. In some musicological studies, they could eventually "discover" that some specific composer could use mv and sv without any disctinction... though I never heard anything about it. But it's something to study, I think. (Musicologists have discovered that Bach used Tr. and mordent notation with no disctinction... they mean exactly the same. =D )