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."