*Singsong voice* Sokka! I think this one is actually the shortest... But yeah, I just saw Sokka playing bass because he's so chill and harmonica because he's so awesome. Can't you?
Sokka made an important decision when he was a toddler, when Katara was just born and stealing all the attention. He decided that he was never going to let his sister beat him in anything. He was going to be faster and smarter and stronger and all around better. It wasn't that he felt he had to put Katara in her place or anything, or that he was jealous, it was just that he was the oldest and so he had to be more outstanding.
So when Katara started taking singing lessons when she was eleven and he was thirteen, he decided that he was going to learn to play an instrument. Nothing too hard, of course; he wasn't going to overexert himself if she was only learning how to sing. He was tempted to ask for singing lessons also, but then it might seem like he was copying her - which he most definitely was not.Besides, he couldn't sing anyways.
Guitar seemed pretty cool, but bass guitar had two fewer strings, so it was the obvious choice. He spent hours practicing, until he had thick callouses on the tips of his fingers. Around the time he finished bass lessons, Katara started writing her own songs. So, of course, Sokka had to learn to do something else. He was wondering about it while he was wandering around the city with his friend Haru when, lo and behold, there was a street musician absolutely blowing away passersby with his mad skills... on the harmonica. Sokka bought a harmonica and a how-to book that same day, much to Haru's amusement. Sokka ignored him and taught himself to play.
And, when he and Katara were accepted into Atla Music College - "Dad, I've graduated 'real' college and Katara's gotten two years in, let us go, please?"- and me met Zuko, Aang, Suki, and Toph, he knew that the hours spent with bleeding fingers from playing his bass for hours and lack of breath from practicing harmonica incessantly were totally worth it.
So as their fans screamed while they walked on stage, he let out a wolf-whistle and stuck his fisted in the air, then started playing.
And he loved it.