I felt like trying my hand at Cas's lack of understanding. I think I did a decent job... It's super short, but whatevs.
I OWN NOTHING
"Oh, come on! My music's not that bad!"
"Are you kidding me, Sammy? It's like Bon Jovi had a pet monkey that took a shit, that shit had kids and a dog, and that shit's dog took a shit, and that's your music."
Castiel's head turned back and forth between the arguing brothers as he attempted to understand exactly what was going on. He spoke up, interrupting Sam's attempt to defend his taste in music. "I… I don't understand. How could a monkey's feces have a family and a pet, and how could its pet produce feces that became music?"
The brothers froze mid-argument, Sam turning slowly in his seat to stare at the angel in the back seat. Dean's gaze went to the rear-view mirror to lock gazes with Castiel. Several seconds passed before Castiel said calmly, "Dean, you're about to hit a tree…"
"SHIT!" The car flung its occupants around a bit as Dean pulled sharply at the steering wheel, narrowly avoiding what would have been a rather unpleasant accident. Seemingly forgetting their previous argument, Sam began bitching about the quality (or lack thereof) of Dean's driving skills. Then they moved on to whether they really need to stop at the gas station for a crappy piece of pie.
Castiel still did not understand the process that created Sam's music.