I might be a little late to the party here, but I couldn't help myself.
Sotto voce does literally mean "undervoice", but there's much more to it than that; it has a very specific connotation.http://en.wikipedia.org/wiki/Sotto_voce
Rather than merely being a dynamic instruction, it conveys an idea, which is the "lowering of one's voice for emphasis". As mentioned in the article, Galileo's famous utterance "Nonetheless, the Earth does move" is a good example of this. I contend that rather than showcasing his Italian, Chopin used this indication very deliberately. Since you're asking about both mezzo voce and sotto voce, I can only assume you're playing op. 48 no. 1, which contains both markings in that order. The second section, which is marked sotto voce, could not be more appropriately suited to that indication. It overflows with subtle, almost celestial profundity. Dynamically speaking, pianissimo is probably about right for that section, since it becomes piano after the crescendo.