In the YouTube comments, you can see a few of the apps used being called out: iGOG is on drums, Pocket Organ fills in some of the melodies and Guitarist puts in the chords. I can only guess that the auto-tuning on "Feliz Navidad" is done by I Am T-Pain. Very cool, and just the thing to make sure you're ready for the upcoming holiday. Merry Christmas to all!
Thanks to everyone who sent this in