I started my second batch yesterday and tasted a little of the early fermentation today. Just a tinge of sourness and sweetness to the rice. Which makes me wonder, what about this method of brewing rice wine leads to the sourness, a flavor I rather like but never detect in commercial sake?
Well, there are many varieties of sour sake in Japan that have concominant lactobacillus infections. Basically, the master sake brewers create a balance, or use a sour mash method (as with moto, if I remember correctly). I've had some more sour "amazake" style wines, and some makkgeoli (korean "fresh" unpasteurized rice wine) had the same tartness. If the lactobacillus is already present in the rice ball, it'll be harder to prevent the sourness. Lactobacillus is inhibited by the presence of lactic acid, so some use that method. Other's claim you can use lysozyme. All of these things are hard to regulate still sometimes, especially in the case of sake if you are growing your own koji (rice ball equivalent-ish).
Alternatively, Chinese rice wine usually begins, in oversimplified and overgeneralized terms (sorry, my chinese friends would kill me if I didn't say that), with the rice cake you see, which is just a mixture of all the fungus and bacterial cultures from a previously inoculated batch of rice that is allowed to grow. This, in turn, is used like a starter to help achieve some form of balance, is what they told me when I asked there. Unfortunately, quality control isn't always so great from batch to batch sometimes, so you get decent variation in quality of product even within the same brand of rice balls (at least in my experience with side by side mason jars).
The Vietnamese do the same general process, but I've had better luck with some of their brands that are made out in California. They use these to not only make wine, but also several desserts with the black sticky rice (which some noted earlier did not fall apart). They treat it like a semi sweet rice pudding-esque dish. It's a real pain to get it right, and my friends keep claiming mine comes out "too alcoholic"....>.<. Just can't please some people, eh?
Sorry, think I ADD'd out there. Hope that helps on the sour question. It's lactobacillus with 99% certainty, and I'd bet (assuming your technique is immaculately sterile) its already present in the ball. On the other hand, I've had lactobacillus work its way into some of my koji/aspergillus growing experiments where I knew the fresh rice was sterile....these kind of things just lend themselves to that stuff. I think thats why the chinese started distilling lost of this stuff. That acetone smell adds that "traditional" flavor

(seriously).
Good brewing!
P.S.
If you get really into it, there are some papers I remember popping up on Google Scholar (great tool, not as obvious on google nowadays) regarding the mixtures of fungi and bacteria that you find in the wines from different asian nations, which contribute to the differences in aroma and taste.