What is that melody is a sound effect trending on TikTok and other video platforms. It's based on the voice line from Sigma's ultimate in Overwatch 2. Sigma is a crazy and volatile astrophysicist who gained his power from an orbital experiment that went wrong. He's a hero that controls gravity, and his ultimate carries enemies into the air, slams them down, and more.
When Sigma carries out his ultimate, he hears and muses over a piano music that plays. He thinks the universe is singing to him and believes it's part of his dementia. The resulting audio is wild and harsh, but follows no actual melody. It may get louder or softer, or even stop altogether. That's what makes it so popular as a meme.