Do we know how/why Nords can speak the dragon language, since that's what thu'um seems to be? It's stated elsewhere that the Voice was the gift of Kyne, right? So what's her association with dragons, if any? Why did Kyne teach the Nords how to speak Dragonish (Draconic?)?

Maybe Kyne taught the giants thu'um for some reason, and the Nords know it because they're apparently descended from giants?

Unless we know/can say for certain that Giants are aspects of Dragons or indeed their descendants (and also leading to connotations of Dragonborns etc.) or something like that then we cannot be 100% sure.

Unless, like the new info coming though suggests, Draconic used to be a widespread language at some point and a lot of creatures can use it. The voice that carries power may have been just an enhanced version of this, trainable etc etc. So Giants learned it back then and when they split into the Giants we encounter today and the Nords, this language remained with them. And language evolves all the time so only a few select people can now use the old language "Thu'um" with deafening results!

Maybe the dragons were once benevolent creatures, possibly created by Kyne? They could have taught the giants/Nord ancestors their language and voice techniques.