In this video some guy, who calls himself One Man Sho, sings gibberish while performing various actions such as knocking over wooden blocks and drinking a glass of orange soda. Then the video plays in reverse to reveal that he was actually singing the national anthem backwards.
It would be possible to fake this. For instance, you could dub over the voice, though I imagine it would be a technical challenge to match the dubbed voice to the movement of his lips. Or you could sing gibberish and then dub over the reversed portion of the video. So I downloaded the video to check that the second half really is the first half played in reverse. It is.
So I'm concluding that the video is real. This guy actually memorized how to sing the national anthem backwards. An impressive, but odd talent.