Music is a huge part of my writing process, so I've had a lot of time to think about this. For my intro, it would have to be Blues Saracceno's "The River".
Why: my story is a Dark Fantasy with Seinen themes blended with Western. The main character is from the American Southwest and is my own take on a Western Gunslinger. When I think of a song that best sums up his journey, especially in the beginning, this is the music I think of. The melancholy tone, the foreboding track, the gritty foreboding tone of the lyrics. To me, this track best summarizes the initial mood of the story.
Now as for the outro, that one's a bit trickier. If I had to choose one I'd probably pick this piece here.
Again, still dark and a little gritty. But it's far more melodic and there's an element of peace that it brings. Almost like the end to a hard struggle, whereupon the hero finds himself resting now. The war isn't one, but for the moment, he can rest his eyes and relax. I see my main character smoking a cigarette in this scene, gazing out the window, his guitar in hand. He takes a long drag from his smoke and blows a cloud of smoke in the air, which forms into the shape of a chinese dragon as the credits roll through it.