Analysis of english three houses theme:
The song is very clearly being sung by Edelgard. Not only did Cristina Vee(Edelgard’s VA) confirm it, there are lines in the song(I look to you, like a red rose). However, there are a few lines that seem to come from Sothis instead (for just this moment lost in time, I am finally me, and the end of the E3 trailer(I will return someday, my teacher, with dragon loli in the background)). So, either we have multiple dragons running around, or Edelgard is actually Sothis. The line “Yet here I hide, behind this mask that I have become.” suggests that this is the case, that Edelgard is an illusion. Edelgard has also been much more prominent than the other 2 house leaders in trailers, suggesting that she will have a major role in the main story. The lines “Gray lights pass through colored glass in this beloved place, silver shines, (the world dines?), a smile on each face” seem to be talking about the academy, which means that Sothis likely feels at ease at the academy before everything goes to shit. The line “My blackened heart, scorched by flames, a force I can’t run from” could be referring to Edelgard becoming empress of the Adrestian Empire. Perhaps the Adrestian empire is the big bad, and Edelgard doesn’t want to become its leader, while blackened heart may refer to Sothis, or stuff Edelgard has seen in the Adrestian Empire. Of course, the “sun” is probably Byleth, but since Sothis is appearing in Byleth’s dream, it gives further evidence that Edelgard is Sothis.
TL;DR: Edelgard is Sothis in disguise