So we know that they have a record of every query that Alexa has detected. I'm guessing what you're questioning is whether they're storing the actual voices so that they can differentiate between different people that have queried Alexa.
I'm quite confident that they store the voice data. It's very valuable to them as a source of training data for future versions of 'Alexa'-like products.
Definitely, as well as being able to eventually differentiate between mom, dad, and the kids.