And what do ninjas play? Well, when the ninja in question is Scorpion from Mortal Kombat, they play the theme to Mortal Kombat, naturally.
It's kind of impressive how easily Scorpion reels off a recognizable version of this cheesetastic old-school techno "classic" on his trusty accordion. hell, we didn't even know Scorpion played the accordion, and it's hard to imagine him finding time to practice between all that ninja training and competing in the Mortal Kombat tournament.
Maybe that explains the far-off look in his eye he displays as he plays -- he's busy imagining the sweet-ass fatality he's going to perform on Raiden as soon as he finishes the last few bars of the tune... And now we can't stop imagining how cool it would have been if he would have had an accordion-based fatality in the game and/or movie.