You can get really great quality voice audio taking as little as 2KB/s(I work with VOIP solutions so I have some idea). At that rate, that's 168MB/24 hours. But then if you consider that you only need to record when someone is actually talking(don't record below certain threshold) you probably only need to store 5-10MB of data a day. You can just write that much to ram, I'm sure Echo has enough to do that. Then upload data periodically when someone sends an actual command to the server.