If somebody is as ignorant as me as to what is this about, it’s about ETH cryptocurrency changing to Proof of Stake, take it from what seems to be the original source:
https://ethereum.org/en/upgrades/merge/
Maybe there is more to it but isn’t that just a wordplay wherein sound means both what music creates and a word signifying something making sense? Or so you mean something related to the supply?
Yes, there is more to it. "Soundness" is a specific term in computer science regarding a program being unable to end up in a invalid state. Usually used in type systems to describe a type system that makes your program unable to be "wrong".
> Ultra sound money is an Ethereum meme focusing on the likely decrease of the ETH supply.
> If capped-supply gold is sound, decreasing-supply ether is ultra sound.
I have no idea what this last sentence means.