I have always just stored unused seed back in its original bag and stored it in my insulated barn. Still, germination rates of older soybean seed diminishes rapidly - more so than any other seed IMO. I typically purchase my soybean seed on the Conservation Seed Program from either NWTF or NDA so it is already a year old when I get it. If I don't use it all that year I absolutely want to use it all by the following year, and even then I would expect reduced germination rates.
It's a crap shoot with soybean seeds. They dry out quickly. Keep them in a cool semi-humid environment. I don't know what the standard procedure is today, but back in the day, unsold, untreated soybean seed went to the feed mill.