The episode "i Remember You" storyboarded by Rebecca and Cole Sanchez is almost half songs. Two of AT's favorite characters (after Finn and Jake of course) are Marceline and The Ice King, but until now, they haven't had much to do with each other. Now you'll know why.
The instrument you hear in this demo is an Omnichord, a kind of electronic autoharp that was made by the Suzuki Instrument Corporation in the 80s. Everyone liked the sound of it so much, that it is Rebecca's actual recording on the soundtrack. The off-key notes were accidents of running out of power—it took a lot of tries to make it happen on cue.