The teaser for the newThe Legend of Zelda, a sequel to Breath of the Wild, was short, but it was enough for fans to investigate and discover new things.
The discovery in question: when the "music" playing in the background of the trailer is reversed, its melody has the same notes as the Game Over theme from the first The Legend of Zelda on NES.
Although the reversed audio is dark, the similarity is not very obvious, but Twitter user @EntranceJew made a video comparing the two melodies more clearly, making everything easier to hear.
https://twitter.com/EntranceJew/status/1138784740113354758
It is worth noting that the series is already accustomed to using this kind of device: the song Ballad of the Goddess, the main theme of The Legend of Zelda - Skyward Sword, contains the melody of Zelda's Lullaby, a classic piece from the franchise.
This mechanic has also been used in the games themselves, with the "Inverted Song of Time" in The Legend of Zelda - Majora's Mask. In the game, playing the inverted version of the "Song of Time" melody on his Ocarina makes time pass more slowly for Link, making his journey easier.

Like the teaser, Majora's Mask also has darker themes
So far, we do not know whether the hidden message in the reversed teaser will serve a purpose in "Breath of the Wild 2", but it certainly adds to the dark atmosphere the team wants to bring to the title. Other people claim to hear intelligible phrases among the whispers in the audio, but there is no confirmation of that for now.