BtVS/Angel by Whedon and Mutant Enemy.
I think I might owe Illyria's nickname to another work I can't find right now... Apollo's one of Greece's complicated gods. His domains include oracles and poetry but he has his negative traits, including being a vengeance seeking sore winner. I frankly feel the pantheon needs a god of therapists... might have helped save the life of one of his sons.
Illyria, mourning those lost in the final battle against Wolfram and Hart, turned to music more human than that of the Song of the Green.
In a few months she had a top-selling album under a stage name, although certain whispered circles referred to her as the 'God-King of the Blues.'
Hearing that her private performances could break hearts and show a person's soul, Apollo, god of music, appeared before her and challenged her to a duel. Her harmonica versus his guitar.
Illyria was pleased to be facing someone more interesting than the rest of the 'muck.' "Any restriction as to the choice of music?"
Apollo smiled, remembering how he'd defeated a satyr. "No, the song doesn't matter. The only rule is that we have to play it while holding our instruments upside down and singing at the same time."
Smiling coldly, Illyria examined her hand. The armor that covered most of her body lengthened to completely cover her fingers. With a crackling noise, she modified the part above her wrist to include a set of intake valves and extended hollow tubes above her fingertips, so she could push air through. "You're on," she said, determined to win.