Everyone seems to assume that Sonasoft would have deleted whatever information it had gotten from the IRS at that time. That is certainly a logical assumption; in fact, it would make sense to require Sonasoft to get rid of any customer’s data once the business relationship ends. But it wouldn’t hurt for a House committee to lay a subpoena on Sonasoft to learn more about the IRS’s dealings with that company and make certain that it doesn’t still have any IRS records.
I don't assume they are gone. More than five years after I stopped using a particular Internet service, they still host a 1997 paper I wrote that I uploaded to the site before I cancelled that service.
Unless the IRS specifically told Sonasoft to delete and scrub everything, this seems like a possible source.