Because sound money is defined simply as stable, you're unlikely to gain just by saving because you normally have to account for inflation. "Healthy" inflation amounts to just about 2% annually, so you'll probably lose that much if you just hoard sound money.

Bitcoin isn't sound money by definition, but it's awfully close to what you've described. Hold it and it grows in value, and if you can predict price movements and trade properly, you can make it grow even faster.