Ever since Levi was a few months old, Jarrod would hum this tune he made up to him. He may not have actually made it up, it's familiar sounding, but if it comes from somewhere else we don't know where.
Jarrod usually puts different words to it. It's usually "Levi, baby boy, Levi little baby boy," but the lyrics often change with whatever's going on.
Levi recognizes his song. Jarrod will sometimes play it for him on the tuba, and he just goes cheesy happy! Jarrod can be in the middle of playing something else and Levi will be playing with his toys - and as soon as Jarrod starts playing this song, he wildly turns around and dances or smiles. He can be in the middle of whining - but when "his" song starts playing his mood does a complete 180 (as you'll see in the following clip).
Here's Levi being really tired and whiny while Jarrod is practicing. I asked Jarrod to play Levi's song, and took a video of Levi's response. This happens 100% of the time: