Tenacious D in The Pick of Destiny/WMG

Kage and Jables actually won the Rock-Off.
Satan violating the Demon Code by not realizing he'd been beaten weakened him enough for the banishment to work. I mean, that was a verifable Crowning Music of Awesome.

The song 'Beelzeboss' is actually what The D sing about in the song 'Tribute' with a bit of musical license added to it.
When trying to remember the song in the post credits sequence, KG describes it as 'The Greatest Song in the World'.
 * That was the general implication. They also said they couldn't remember the song.

'Tribute' was actually about a revenge attempt by Satan, who sent a weaker, less rocking demon to fight them.
Because several things about the song contradict the movie. The demon didn't appear in front of them to challenge them, they summoned him, and he reluctantly accepted their challenge because of the demon code. He also was not impressed by their song, but instead declared it lame. Since Satan can no longer appear in the human world, he had to send one of his minions to do his bidding. Being less rocking, he was stunned by the songs that they played.

Satan was never challenged to a rock-off before.
During his part in the rock-off, he sings that there's never been a rock-off that he's ever lost. But if that was because he always won, his reaction when being challenged would've been contained and even smug. Instead, he got mad and cussed because he never took part in any rock-off before, and that's the only reason why he never lost.