If you still don't believe me and think that they just slapped over a pre-recorded version of the audio, you can watch their performance on Ellen (http://www.youtube.com/watch?v=SvviLGNydeg)
I browsed through Walk Off the Earth's YouTube channel and found their other covers as well as their original songs. So far I have not heard anything that I didn't like. I especially like their covers of "From Me To You" by The Beatles and "Party Rock" by LMFAO. As for original songs so far I've listened and liked "Stolen" and "Money Tree".
Go visit their YouTube channel and listen/watch to their other videos.