I suppose a lot of arguments could emerge concerning the term, sound money. Apologies for a brain dump of my understanding that follows. Money, in my limited and probably poor understanding and have managed to scrape from google and various books, is merely a store of value.
Let's say that others use a different definition for money. That's ok. For purposes of what follows, my meaning is a store of value.
From what I understand, the first money was seeds, and eventually certain metals that were less corruptible than others and more difficult to find than more common materials such as sand.
Sand can be turned into glass, and glass is arguably longer lasting and less corruptible than gold, so long as it doesn't get smashed. But, it is also more available - or rather, easier to devalue existing stores of value.
Perhaps the other aspect is some intrinsic value, where value means something more than just wavering psychology, subject to a limitations in supply and the anticipation of increased speculative value, but rather something that can be used independent of belief system, having some utility in and of itself, for forming useful objects that aren't as corruptible.
Unfortunately, it seems that due to the relatively small amounts of gold, it is easier for others to mess with the value by manipulating the market for gold itself, which sort of limits its utility as a store of value, unless one is willing to accept that the value can be all over the place.
From what little I understand of bitcoin, one of the things that gives it value is that it can be redeemed in the currency of various governments over the world. From what I can tell, most of the value is more of the psychological kind, having limited utility in and of itself, other than perhaps to some degree, sidestepping the banks and such.
I suppose the value of the fiat currency is really how many things or services can you buy with it and how long does it hold the value, or even, how predictable is its value curve. From what I understand, the original US dollar was actually based on some Spanish and other coins due to their wide availability and acceptance in the colonies.
I understand that the paper dollar is somehow designed to lose value. If the decline in value is at least somewhat predictable, and the supply not easily manipulated then maybe it is predictable enough to use as a store of value. Can the same be said of bitcoin? From what I can tell, its value is fairly unstable, but is it more unstable than the US dollar?
Maybe that is the real question when trying to find a place to store value or discussing, sound money.
Personally, I suspect money should somehow be tied to land, but have not given any thought to implementation.