I googled it for you and got back:
"Sound money is a phrase technically synonymous with a currency backed by or pegged to some good or commodity. It can also mean a currency that is not rapidly inflating or deflating in value. As even the technical definition is rather fuzzy, in reality, it's often synonymous with whatever economic woo its promoter is hyping. Sound money is often a cover for gold buggery, though it can mean whatever the hell the bullshit purveyor wants it to mean."
Did you get that from here?
http://rationalwiki.org/wiki/Sound_moneySeems like a very biased article. For example does sound money really need to be "backed by or pegged to some good or commodity".
In my view that article isn't very credible at all; seems more like propaganda.
I was hoping for a definition that says something like:
sound money is a currency that cannot be debased or counterfeited...