Here is the part we hear her humming:
Spoilers for part 2 of that episode.
It's the song the Ophidians (snake-men) were singing when creating the Heart of Darkness. You can hear them chanting in the video below (2'55), and Mophir talks about the "dark words" of the crystal. Given that those who touch the crystal become possessed by revengeful Ophidians spirits/magic/stuff, it seems logical they'd sing an Ophidian tune.
I'm not aware of a specific name for that song.